RODENTIA, or Glires, an order of placental mammals characterized by the peculiar form and structure of their front or incisor teeth, which are reduced to a single functional chisel like pair in each jaw, specially adapted for gnawing, and growing throughout the entire life of their owners. Rodents may be characterized as terrestrial, or in some cases arboreal or aquatic, placental mammals of small or medium size, with a milk and a permanent series of teeth, plantigrade or partially plantigrade, and generally five-toed, clawed (rarely nailed or semi-hoofed) feet, clavicles or collar-bones (occasionally imperfect or rudimentary), no canine teeth, and a single pair of lower incisors, opposed by only one similar and functional pair in the upper jaw.
In all rodents the upper incisors resemble the lower ones in growing uninterruptedly from persistent pulps, and (except in the hare group, Duplicidentata) agree with them in number. The premolars and molars may be rooted or rootless, with tuberculated or laminated crowns, and are arranged in an unbroken series. The orbits are always open behind, never being surrounded by bone. The condyle of the lower jaw is antero-posteriorly elongated. The intestine (except in the dormice or Gliridae) has a large caecum. The testes are inguinal or abdominal. The uterus is two-horned, with the cornua opening separately into the vagina or uniting to form a corpus uteri. The placenta is discoidal and deciduate. And the smooth hemispheres of the brain do not extend backwards so as to cover any part of the cerebellum.
Rodents include by far the greater number of species, and have the widest distribution, of any of the orders of terrestrial mammals, being in fact cosmopolitan, although more abundant in some parts, as in South America, which may be considered their headquarters, than in others, as in Australasia and Madagascar, where they are represented only by members of the mouse-group, or Myoidea.
All rodents are vegetable-feeders, and this uniformity in their food and in the mode of obtaining it, namely by gnawing, has led to that general uniformity in structure observable throughout the group; a feature which renders their classification difficult. Indeed, despite the fact that they present much diversity of habit—some being arboreal, as the squirrels, many of which are provided with expansions of skin or parachutes on which they glide from tree to tree; some cursorial, as the hares; others jumpers, as the jerboas; others fossorial, as the mole-rats; and others aquatic, as the beavers and water-rats—no important structural modifications are correlated with such diversity of habit.
Anatomy.—The rodent skull is characterized by the great size of the premaxillae, which completely separate the nasals from the maxillae; by the presence of zygomatic arches; and by the wide unoccupied space existing between the incisors and the cheek-teeth; and (except in the Duplicidentata) by the antero-posteriorly elongated glenoid cavity for the articulation of the lower jaw. Post-orbital processes of the frontals exist in squirrels, marmots and hares; but in all other genera they are rudimentary or altogether absent; and the zygoma seldom sends upwards a corresponding process, so that the orbit is more or less completely continuous with the temporal fossa. The lachrymal foramen is always within the orbital margin; and in many species the infra-orbital foramen is very large (in some as large as the orbit) and transmits part of the masseter muscle. The zygomatic arch is variously developed, and the position of the jugal is a character for grouping the families. The nasals are, with few exceptions, large, and extend far forwards, the parietals are moderate, and there is generally a distinct interparietal. The palate is narrow from before backwards, this being especially the case in the hares, where it is reduced to a mere bridge between the premolars; in others, as in the rodent-moles (Bathyerginae), it is extremely narrow transversely, its width being less than that of one of the molar teeth. Tympanic bullae are always present and generally large; in some genera, as in the gerbils (Gerbillinae) and jerboas (Jaculidae), there are supplemental mastoid bullae which form great hemispherical bony swellings at the back of the skull. (fig. 1, Per), in these genera and the hares the meatus auditoriums being tubular and directed upwards and backwards. The lower jaw is characterized by its abruptly narrowed and rounded front part supporting Fig. 2.—Skull of Porcupine (Hystrix cristata), with muscle attached. t, temporal muscle; m, masseter; m′, portion of masseter transmitted through the infra-orbital foramen, the superior maxillary nerve passing outwards between it and the maxilla. the pair of large incisors, as well as by the small size of the coronoid process, and the great development of the lower hind, or angular, portion.
The dental formula varies from i. 2, c. 0, p. 3, m. 3 (total 28) in the hares and rabbits to i. 1, c. 0, p. 0, m. 2 (total 12) in the Australian water-rats; but in the great majority of species it presents striking uniformity, and may be set down typically as i. 1, c. 0, p. 1 or 0, m. 3. In the Duplicidentata only is there more than a single pair of incisors, and in these the additional pair is small and placed behind the middle pair. In this group the enamel extends partially to the back of the incisors, but in all the rest it is restricted to the front surface, so that, by the more rapid wearing-away of the softer structures behind, a chisel-shaped edge is maintained. Both upper and lower incisors are regularly curved, the upper ones slightly more so than the lower; and, their growth being continuous, should anything prevent the normal wear by which their length is regulated—as by the loss of one of them, or by displacement owing to a broken jaw or other cause—the unopposed incisor may gradually curve upon itself until a complete circle or more has been formed, the tooth sometimes passing through some part of the animal's head. The cheek-teeth may be either rooted or rootless, and either cusped or formed of parallel plates, this diversity of structure often occurring in the same family. When there are more than three cheek-teeth, those which precede the last three have succeeded milk-teeth, and are premolars. In some species, as in the agoutis (Dasyproctidae), the milk-teeth are long retained, while in the allied cavies (Caviidae) they are shed before birth.
Fig. 3.—Vertical and Longitudinal Section through the Skull of the Beaver (Castor fiber), showing the brain-cavity, the greatly developed plates of bone in the nose-cavity, the mode of implantation of the ever-growing chisel-edged incisor, and the curved rootless cheek-teeth.
The tongue presents little variability in length, being short and compressed, with a blunt tip, which is never protruded beyond the incisors. In most species there are three circumvallate papillae at the base, and the apical portion is generally covered with small thread-like papillae, some of which in the porcupines become greatly enlarged, forming toothed spines. The stomach varies in form from the simple oval bag of the squirrels to the complex ruminant-like organ of the lemmings. In the water-rat and agoutis it is constricted between the oesophagus and pylorus; while in the dormouse the oesophagus immediately before entering the stomach is much dilated, forming a large egg-shaped bag with thickened glandular walls; and in certain other species, as in Lophiomys and the beaver, glandular masses are attached to and open into the cardiac or pyloric pouches. All rodents, with the sole exception of the dormice, have a caecum, often of great length and sacculated, as in hares, the water-rat and porcupines; and the long colon in some, as the hamster and water-rat, is spirally twisted upon itself near the commencement. The liver is divided in the typical manner in all, but the lobes are variously subdivided in different species (in Capromys they are divided into minute lobules); and the gall-bladder, though present in most, is absent in a few. In most species the penis (which is generally provided with a bone) may be more or less completely retracted within the fold of integument surrounding the vent, and lie curved backwards upon itself under cover of the integument, or it may be carried forward some distance in front of the anal orifice, from which, as in voles and marmots, in the breeding-season, it is separated by the prominent testicular mass. The testes in the pairing-season form projections in the groins, but (except in the Duplicidentata) do not completely leave the cavity of the abdomen. Prostate glands and, except in the Duplicidentata, vesiculae seminales are present in all. The uterus may be double, each division opening by a separate os uteri into a common vagina, as in Leporidae, Sciuridae, and Hydrochoerus, or two-horned, as in most species. The teats vary in number from a single abdominal pair in the guinea-pig to six thoracic-abdominal pairs in the rats; while in the Octadontidae and Capromyidae they are placed high up on the sides of the body.
There are generally nineteen dorso-lumbar vertebrae (thirteen thoracic and six lumbar), the form of which varies in different genera; in the cursorial and leaping species the lumbar transverse processes are generally very long, and in the hares there are large compressed inferior spines, or hypapophyses. The caudal vertebrae vary from a rudimentary condition in the guinea-pig to a great size in the jumping-hare and prehensile-tailed porcupines. The scapula is usually narrow, with a long acromion; the clavicles may be altogether absent or imperfect, as in porcupines, cavies and hares, but in most species are well developed. The humerus has no supra-condylar foramen, and the forearm bones are distinct; and in most species the fore foot has five digits with the phalanges normally developed, the first toe being but rarely rudimentary or absent. The pelvis has large ischia and pubes, with a long and usually bony symphysis. The femur varies considerably in form, but generally has a well-defined third trochanter. In the squirrels and porcupines the tibia and fibula are distinct, but in rats and hares they are united, often high up. The hind foot is more variable than the front one, the digits varying in number from five, as in squirrels and rats, to four, as in hares, or even three, as in the capybara, viscacha and agouti. In the Jaculidae the metatarsals are greatly elongated, and in some of the species, as jerboas, they are welded together.
The mouth is divided into two cavities communicating by a narrow orifice, the anterior one containing the incisors and the posterior the molars, the hairy skin of the face being continued inwards behind the incisors. This evidently prevents substances not intended for food getting into the mouth, as when the animal is engaged in gnawing through an obstacle. In hares and pacas the inside of the cheeks is hairy; and in some species, pouched rats and hamsters, there are large internal cheek-pouches lined with hair, which open near the angles of the mouth and extend backwards behind the ears. In the New World pouched rats (Geomyidae) the pouches open externally on the cheeks.
The peculiar odour evolved by many rodents is due to the secretions of special glands, which may open into the prepuce, as in Mus, Microtus and Cricetus, or into the rectum, as in Arctomys and Thryonomys, or into the passage common to both, as in the beaver, or into pouches opening near the vent, as in hares, agoutis and jerboas.
The skin is generally thin, and the panniculus carnosus muscle rarely much developed. The fur varies exceedingly in character,—in some, like the chinchillas and hares, being fine and soft, while in others it is more or less replaced by spines on the upper surface, as in spiny rats and porcupines; these spines in several genera, as Xerus, Acomys, Platocanthomys, Echinothrix, Loncheres and Echinomys, being fattened. In muscular structure the chief peculiarities are noticeable in the comparatively small size of the temporal muscles, and in the great double masseters (fig. 2), which are the principal agents in gnawing. The digastric muscles also are remarkable for their well-defined central tendon, and in many species their anterior bellies are united between the two halves of the lower jaw. The cleido-mastoid generally arises from the basi-occipital, and the pectoral is major is connected with the latissimus dorsi. In porcupines and hares the tendons of the flexor digitorum longus and flexor hallucis longus are connected in the foot, while in the rats and squirrels they are separate, and the flexor digitorum longus is generally inserted into the metatarsal of the first toe.
Classification.—Some diversity of view obtains among naturalists with regard to the classification of the order; the scheme here followed being the one adopted (with some modifications of nomenclature) by Professor Max Weber in his Säugethiere. The number of genera is so great that only the more important can be noticed. All authorities are agreed in dividing rodents into two great sections or sub-orders, the one, Duplicidentata, comprising only the hares, rabbits and picas, and the other, Simplicidentata, all the rest. In the latter there is only one pair of incisor teeth in the upper jaw, in which the enamel is confined to the front surface. The incisive foramina of the palate are moderate and distinct; the fibula does not articulate with the calcaneum; and the testes are abdominal, and descend periodically only into the inguinal canal.
Sewellels.—The first family is represented by certain peculiar North American rodents known as sewellels, constituting the genus Haplodon (or Aplodon) and the family Haplodontidae and section Haplodontoidea. In common with the next three sections these rodents have the angular process of the lower jaw (fig. 4) arising from the inferior surface of the socket of the incisor. The masseter muscle does not pass through the narrow infra-orbital canal. An alisphenoid canal may be present on the palatal aspect of the skull; but there is always a transverse canal. The malleus and incus of the inner ear are separate. The humerus often has a foramen (entepicondylar) on the inner side of its lower end; the tibia and fibula may be separate or united; but the scaphoid and lunar of the carpus are also united, while the centrale is free. The stomach is simple.
Fig. 4.—Skull of the American Marmot (Arctomys monax). The projection at the right-hand lower corner of the figure is the angular process of the lower jaw.
Sewellels are medium-sized terrestrial rodents, with no post-orbital process to the skull, which is depressed in form, and rootless cheek-teeth, among which the premolars number 2, the first in the upper jaw being very small. The build is stout and heavy, the limbs and tail are short, the ears moderate, the eyes minute and the feet five-toed and plantigrade. Haplodon is represented by a small number of species in America west of the Rocky Mountains, of which H. rufus is the longest known. They are burrowing, and, in some cases at any rate, partially aquatic rodents.
Squirrel Group.—The Sciuroidea, which include the great group of squirrels, sousliks, marmots, &c., all comprised in the single family Sciuridae, differ from the sewellels in having large post-orbital processes to the skull (figs. 4, 5, 6); and, with one exception, have rooted cheek-teeth, the premolar-formula being 2 or 1. The infra-orbital foramen is also narrower, and the tympanic bulla is cellular. In both groups the tibia and fibula are separate.
The family is divided into three sub-families, the first of which is the Sciurinae. In this the crowns of the molars are more or less shortened, with their cusps either arranged in longitudinal lines, or forming four upper and three lower more or less distinct oblique ridges. The post-orbital processes of the frontal and jugal are widely sundered, and the former may even be small (Xerus). The expanded anterior root of the zygomatic process has its front border oblique. According to modern views the sub-family is broken up into a large number of genera.
The first of these is Rhithrosciurus, represented by one large species (R. notatus) from Borneo, characterized by its finely grooved incisors (see Groove-Toothed Squirrel). The second genus, Heliosciurus, includes arboreal African squirrels, typified by H. stangeri, allied in the characters of their skulls to the under-mentioned Xerus, and with a very large pre-orbital foramen in the more typical forms. The third, Funisciurus, of which F. pyrrhopus is a well-known example, is also African and allied to Xerus, but has a still longer skull and soft fur. In Xerus itself, which is represented by the terrestrial African spiny squirrels, the ears are short, there are only two teats, and flat spines are mingled with the fur; while the skull, and more especially the frontals, is elongated, with a very short post-orbital process, and the crowns of the molars are taller than usual (see Spiny Squirrel). The well-known Indian palm-squirrel, Funambulus palmarum, typifies an Indo-Malay genus allied to Xerus in skull-characters but with molars more like those of Sciurus. In contrast to these small striped species are the giant squirrels of the same region, such as Ratufa indica and R. bicolor, which are very brightly coloured rodents, with Sciurus-like skulls (fig. 5) but extremely short-crowned molars, and only one pair of upper premolars. Next comes the typical Sciurus, including the great bulk of the entire group, and ranging over Europe, Asia, North Africa and America. The skull is short and broad, especially as regards the frontals, with large post-orbital processes (fig. 5), and very generally two upper premolars, making a total of five pairs of upper cheek-teeth, which have crowns of medium height. The teats are either four or six. Squirrels of this and the other arboreal groups have the bodily form slender and agile, the tail long and bushy, the ears well developed, pointed and often tufted; the feet adapted for climbing, the anterior pair with four toes and a rudimentary thumb, and the posterior pair with five toes, all the toes having long, curved and short-pointed claws (see Squirrel). The names Glyphotes and Sciurotamias have been proposed respectively for one Bornean and some four Chinese squirrels. With Tamias (sometimes split into Tamias and Eutamias) we reach the North American striped ground squirrels, or chipmunks, well characterized by the large internal cheek-pouches, with one outlying species in Northern Asia and Europe (see Ground-Squirrel). These lead on to the sousliks, Spermophilus (or Citellus), in which the incisors (as in the following genera) differ from those of all the squirrels in not being compressed. The genus which is common to the northern parts of both hemispheres is distinguished by the large cheek-pouches and by the absence or rudimentary condition of the claw of the first hind-toe, resembles Tamias in the slender form of the body, but displays great variation in the length of the tail, which may be a mere stump, or comparatively long. As in the following genera, there are two pairs of premolars, of which the first in this case is small and rounded, while the two series of cheek-teeth are nearly parallel (see Souslik). The prairie-dogs, or prairie-marmots, Cynomys, are a North American group, in which the five-toed forefeet have the claw of the first as large as that of the fifth toe. The skull is heavily built, with the post-orbital processes directed outwards. Dentition (fig. 6) remarkably heavy, the molar teeth differing from those of Spermophilus and Arctomys by having three instead of two transverse grooves on their crowns. First premolar nearly as large as the second. Molar series strongly convergent behind (see Prairie-Marmot). Finally, we have the marmots (Arctomys), which are larger and more heavily built rodents, with short ears, more or less short tails and rudimentary or no cheek-pouches. Fore-feet with the first toe rudimentary and bearing a flat nail. Skull (fig. 4) large and heavy, with the post-orbital process stouter and at right angles to the axis. Incisors broad and powerful. First upper premolar nearly as large as the second. Molar series nearly parallel, scarcely converging behind at all.
Fig. 5.—Under Side of Skull of the Malay Giant Squirrel (Ratufa bicolor).
Fig. 6.—Under Side of Skull of Prairie-Marmot (Cynomys ludovicianus).
The genus is common to the northern half of both hemispheres, and its members, like those of the two preceding groups, burrow and hibernate (see Marmot).
The Nannosciurinae, or second sub-family of Sciuridae, are represented only by the pigmy squirrels (Nannosciurus), characterized by their very short-crowned molars (which approximate to those of dormice in structure) and small premolars, of which the first upper pair is often deciduous, while the upper molars have only three oblique ridges. The front root of the zygomatic arch is nearly vertical, and placed so far back that it is above the second molar, while the orbit—a unique feature among rodents—is almost completely surrounded by bone. The few representatives of this group are all very small rodents, confined to tropical Africa, the Philippines and the Malay islands.
The third and last sub-family, the Pteromyinae, is distinguished from the other two by the presence of a parachute-like fold of skin along the sides of the body, the supporting cartilage of which arises from the carpus or wrist. It includes Sciuropterus, represented by small species from the northern parts of both hemispheres; Pteromys, comprising large flying-squirrels, ranging from India and the Malay countries to Japan, characterized by the long cylindrical tail and large inter-femoral membrane; and Eupetaurus, represented by one very large dark grey, long-tailed and long-haired species from Astor and Gilgit, which differs from all other members of the family by its tall-crowned cheek-teeth (see Flying-Squirrel).
Beavers.—The second section, Castoroidea, of the present group includes only the family Castoridae, represented by the beavers, which are large aquatic rodents characterized by their massive skulls, devoid of post-orbital processes, with the angle of the lower jaw rounded, the molars rootless or semi-rooted, with re-entering enamel-folds, and one pair of premolars above and below. The tibia and fibula are united inferiorly, the tympanic bulla is hollow and the infra-orbital foramen narrow. The single existing genus comprises the European beaver, Castor fiber, of Europe and Northern Asia, and the North American C. canadensis. The upper molars are subequal, each with one internal and two external enamel-folds; the stomach has a large glandular mass situated to the right of the esophageal orifice; the anal and urino-genital orifices open within a common cloaca; the tail is broad, horizontally flattened and naked; and the hind-feet are webbed (see Beaver).
Pouched Rats.—The American pouched rats, or pocket-gophers, constitute the third section, Geomyoidea, with the single family Geomyidae. The dentition includes one pair of premolars above and below, and rooted or rootless molars with but few enamel folds. In the skull the infra-orbital foramen is narrow, and post-orbital processes and an alisphenoid canal are absent. The tibia are fibula are united. The cheeks are provided with large pouches opening externally. Two sub-families are recognized. The first of these, or Geomyinae, is characterized as follows: Incisors broad; mastoid not appearing on the top of the skull; eyes small; ears rudimentary; limbs short, subequal. Habits fossorial. Geomys bursarius, the “red pocket-gopher” of North America, with deeply grooved incisors, inhabits the plains of the Mississippi, living in burrows like the mole. Several other species from the Southern States, Mexico and Central America are recognized. Thomomys talpoides, with plain incisors, extending from Canada to the United States west of the Rocky Mountains, typifies the second genus, which has also many species. The following are the characters of the second sub-family, Heteromyinae: Incisors narrow; mastoid appearing largely on the top of the skull; eyes and ears moderate or large; hind-limbs and tail elongated. Habits terrestrial. Dipodomys, which has the molars rootless, is typified by D. phillipi, the kangaroo-rat of the desert regions east of the Rocky Mountains, Perodipus and Microdipodops being allied genera. Perognathus and Heteromys have rooted molars; time latter genus is distinguished by the presence of flattened spines among the fur, and has species extending into South America. (See Pocket-Gopher, Pocket-Mouse and Kangaroo-Rat.)
Scaly-tailed Squirrels.—The next section, according to Prof. Max Weber's arrangement, is that of the Anomaluroidea, typified by the rodents commonly called African flying-squirrels (Anomaluridae), but better designated scale-tailed squirrels, or simply “scaly-tails,” since one member of the family has no parachute. To this group Prof. H. Winge affiliates the African jumping-hares (Pedetidae), a view which is adopted by Prof. Weber, although Mr O. Thomas places these rodents in the neighbourhood of the porcupines. In the more extended sense, the Anomaluroidea are diagnosed as follows: In the skull the infra-orbital foramen (or canal) is large, the lachrymal foramen placed high up, and no transverse canal; while the malleus and incus of the internal ear are fused. In the carpus the scaphoid and lunar bones are united. There is a single pair of premolars in each jaw.
The Anomaluridae are characterized by having rooted cheek-teeth with shallow transverse enamel-folds, the two halves of the lower jaw movably articulated in front, very small post-orbital processes to the skull, and the presence of two rows of scales on the under surface of the base of the tail (figs. 7 and 8), which is cylindrical and thickly haired. The family is confined to the equatorial forest-tract of Africa, where it is most numerously represented on the west side. The majority of the species belong to the typical genus Anomalurus (fig. 7), which is provided with a parachute supported by a cartilaginous process arising from the olecranon of the ulna, and has well-developed ears and a moderately long tail. Several of the species are considerably larger than an ordinary squirrel. Idiurus, as represented by the West African I. zenkeri (figured in the article Flying-Squirrel), is a mouse-like form, with very small ears and an extremely long tail. The third genus, Zenkerella (Aëthurus), which is also West African, has no parachute (fig. 8).
Jumping-Hares.—The grounds for referring the African jumping-hares (Pedetidae) to the Anomaluroidea rest largely on the evidence of certain Tertiary rodents from Europe, such as Issiodoromys. The family is represented by the South African Pedetes caffer, which is as large as a hare, and the smaller East African P. surdaster. In general habits and appearance these animals recall large jerboas, from which group they are, however, distinguished by the four pairs of rooted cheek-teeth, the premolars being as large as the molars, and the latter having one outer and one inner enamel-fold. The hind-limbs are elongated, with four toes, of which the metatarsals are separate; the tibia and fibula are welded in old age; the calcaneum and astragalus of the tarsus are elongated; and there is a perforation on the inner side of the lower end of the humerus (see Jumping-Hare).
|Fig. 7.—Red Scaly-tailed Squirrel (Anomalurus fulgens).|
|From de Winton.|
Fig. 8.—Zenker's Scaly-tailed Squirrel (Zenkerella insignis).
Dormice.—The next three sections of the order, namely, the Myoxoidea, or dormice, Dipodoidea, or jerboas, and Myoidea, or the mouse group, have the following characteristics in common. The angular process of the lower jaw has the same relations as in the sewellels and the allied groups. The lachrymal foramen in the skull is low down and forms an elongated slit. In the carpus the scaphoid and lunar are welded, but the centrale remains distinct. The tibia and fibula are fused at their upper and lower ends. The malleus and incus of the inner ear are separate. Except in Lophiomys, the clavicles are complete. The infra-orbital foramen of the skull (fig. 9) is more or less broad; and there is generally a transverse canal. The stomach is generally complex.
In the dormice, forming the section Myoxidea, with the single family Gliridae (or Myoxidae), a single pair of premolars may or may not be present; the molars are short-crowned and rooted, with transverse enamel-folds. The angle of the lower jaw is twisted and its coronoid process slender. Dormice are small arboreal rodents, with long hairy tails, large eyes and ears, and short fore-limbs, ranging over Europe, Asia and Africa. Of the four genera in the typical sub-family Glirinae, the first is Glis, represented by Glis vulgaris (or G. glis) of Europe, with a doubly vaned, bushy tail, simple stomach, and large molars with well-marked enamel-folds; the second, Muscardinus, with M. avellanarius, the common dormouse, distinguished by the cylindrical bushy tail, and thickened glandular walls of the cardiac extremity of the œsophagus; thirdly, Eliomys, containing several species, with tufted and doubly vaned tails, simple stomachs and smaller molar teeth, having concave crowns and faintly marked enamel-folds; and lastly, the African Graphiurus, represented by several species, with short cylindrical tails ending in a pencil of hairs, and very small molars almost without trace of enamel-folds. None of the members of the typical sub-family extend into India, where the group is represented by Platacanthomys, typifying the sub-family Platacanthomyinae, characterized by the absence of premolars; the other being the Chinese Typhiomys. These are small rodents with somewhat the appearance of the pigmy squirrels (Nannosciurus), which in some degree connect the family with the Muridae. (See Dormouse.)
Jerboa Group.—The Dipodoidea, or jerboa-group, which likewise includes only a single family, Jaculidae (or Dipodidae), is characterized by the presence of not more than one pair of premolars in the upper jaw, which, however, may be wanting; by the rooted cheek-teeth, which have transverse enamel-folds, and the absence of a transverse canal in the skull, and of a horny layer in the stomach. The family is divisible into two sub-families, of which the first, or Sminthinae, is represented only by the genus Sminthus, containing a few species which range from, Denmark into Western Asia, Kashmir and China. They are small rat-like rodents, with one pair of upper premolars, which are mere pins, as is the last molar, and the two pairs of limbs of normal length, with the metatarsals separate; the infra-orbital opening in the skull being triangular and widest below, while the incisive foramina in the palate are elongated. The European S. subtilis has a black dorsal stripe bordered with yellow.
The Dipodinae, on the other hand, are leaping rodents, with the metatarsals elongated, a small upper premolar present or absent, and the crowns of the molars tall. Various degrees of specialization occur in the adaptation for leaping. The least specialized genus is Zapus, containing the jumping-mice of North America, with one outlying Siberian species, in which the five metatarsals are free, as are also the cervical vertebrae, the small upper premolar being retained. (See Jumping-Mouse.)
In the other genera, so far as known, the three central metatarsals of the hind foot are fused into a cannon-bone, of a type unique among mammals and comparable to that of birds. Some of the cervical vertebrae are also united in at least the better-known genera. The tail and ears are generally very long; while, in correlation with the size of the latter, the auditory bullae of the skull are also large. In the typical jerboas, Jaculus (or Dipus), ranging from North Africa to Persia, Russia and Central Asia, there are only three hind toes, the incisors are grooved, and the premolars are generally wanting. The other genera have five toes, of which only the middle three are functional, and smooth incisors. Euchoreutes, with one Yarkand species, has premolars, enormous ears and a long nose. Alactaga, ranging over Russia and Western and Central Asia, inclusive of Persia and Baluchistan, has smaller ears and a shorter nose; by some naturalists it is taken to include the North African A. tetradactylus, which is separated by others as Scarturus. The Turkestan Platycercomys (or Pygeretmus) has a lancet-shaped tail and no premolars; while Cardiocranus of the Nan-shan district of Central Asia has a similar type of tail, but short ears and a peculiarly triangular skull. (See Jerboa.)
Mole-Rats.—The mole-rats (Spalacidae) bring us to the mouse-like section, or Myoidea, in which there are no premolars and the Fig. 9.—Skull of the Muskrat (Fiber zibethicus). Natural size. molars may be occasionally reduced to 2; these teeth being either rooted or rootless, with either cusps or enamel-folds, and the first generally larger than the second. In the skull the zygomatic arch is slender and the jugal bone small and not extending far forwards, being supported by the long zygomatic process of the maxilla, while the infra-orbital foramen is mostly large, and there are no post-orbital processes. Although sometimes short, the tail is generally long, sparsely haired and scaly. The cardiac portion of the complex stomach has a horny layer, and there is a caecum.
The Spalacidae are burrowing types, allied apparently to the ancestral Jaculidae, and characterized by the second and third molars being equal in size, the presence of enamel-folds in all these teeth, and the superiority in size of the claws of the second, third and fourth front toes over the other two. All these “rodent-moles” are thoroughly adapted to a subterranean life, the eyes and ears being small and rudimentary, as is also the tail; while the bodily form is cylindrical, and the front claws are very large and powerful. The incisors are very large; and the palate of the skull is narrow. The typical representative of the group is the great mole-rat (Spalax typhlus) of Eastern Europe and North-East Africa, which, together with a few closely allied species, has the eyes completely buried in the skin, and the head much flattened. In the bamboo-rats, Rhizomys, from the Indo-Malay countries, China and Tibet, as well as in the closely allied East African Tachyoryctes, the eyes are, however, functional, and the head is rounded. (See Mole-Rat.)
According to the arrangement here followed, the burrowing zokors may be placed in this family, although they have teeth like those of the vole group in the Muridae. The first representative of this sub-group is the genus Siphneus (or Myotalpa), of which some five Central and North Asiatic species are known. They are characterized by the mole-like form and long, powerful, front claws (fig. 10). In the true zokors (Ellobius), on the other hand, the claws are short and the general form more vole-like. Of three named species, one extends from South Russia to Siberia, while two others are respectively from Kurdistan and Afghanistan. A third type, Prometheomys, from the Caucasus, is represented by a species of the size of a small water-rat, chestnut-brown in colour, with lighter feet, and the minute eyes covered with skin. The teeth are nearest to those of the true zokors (Ellobius). The single example was taken under flowering anemones.
|Fig. 10.—The Tibetan Zokor (Siphneus armandi).|
Malagasy Rats.—On account of certain structural peculiarities, the rats of Madagascar, which have a dentition like that of the cricetine Muridae, are separated as a distinct family, Nesomyidae. They are the only rodents in that island. Of these, Hypogeomys is a large, long-tailed, fawn-coloured rat, with large ears and feet; Nesomys is a red species, with long hair; Brachytarsomys is short-footed and long-tailed, with velvety fawn fur; Hallomys has elongated hind feet, as has also Macrotarsomys; Gymnuromys is naked-tailed; and the several species of Eliurus are dormouse-like.
Mouse Tribe.—The characteristics of the Muridae are those of the Myoidea generally, as given above under the heading of the Spalacidae. With the exception of Madagascar, the family, which may be divided into six sub-families, has a cosmopolitan distribution, and the genera are so numerous that only some of the most important can be even mentioned.
The first group is that of the hamsters, or cricetines (Cricetinae), in which the molars are rooted and tuberculated, with the cusps of the upper ones arranged in two longitudinal rows (fig. 13, B); in the upper teeth the outer cusps and in the lower the inner ones are the higher, and when worn the crown surfaces show oblique dentine-areas; in shape the third molar is like the second, but it is smaller. The infra-orbital foramen is generally narrow, and the tympanic bulla hollow. The humerus has a foramen at the lower end. The tail is short. The group is typified by the European hamster (Cricetus vulgaris or C. cricetus), to which a separate article is devoted (see Hamster); the genus includes a number of species ranged under several sub-genera, such as Mesocricetus, Cricetulus, and Urocricetus, widely spread in Western and Central Asia, the last-mentioned, which is from Tibet, being distinguished by its relatively long tail. The hamsters all possess cheek-pouches, which are, however, absent in many of the following genera. Africa claims only a single representative of the group, Mystromys, with one southern and one eastern species. Persia is the home of Calomyscus (with one species), a near relative of the American Peromyscus. In America, where the more typical kinds are known as white-footed, or deer, mice, the cricetines absolutely swarm, and include a host of genera, the majority of which are North American, although others are peculiar to Central and South America. Among these may be named Onychomys, Peromyscus, Rhipidomys, Holochilus (which is South American and includes the largest species), Sigmodon (typified by the North American rice-rat, S. hispidus), Oryzomys, Rhithrodontomys (with grooved incisors), Ichthyomys and Anotomys (fish-eating, aquatic forms, from the mountains of South America), Acodon, and the North American wood-rats, or Neotoma, in which the molars have a structure simulating that of the under-mentioned Microtinae. A distinct sub-family, Lophiomyinae, is represented by the Central African arboreal spiny rats, Lophiomys, of which there are two or three species. Although agreeing with the Cricetinae in the hollow tympanic bullae, they have the clavicles imperfect, the first front toe opposable to the rest, the temporal region of the skull roofed with bone, and the crowns of the molars with cusps arranged in rows but eventually covered by a layer of enamel.
|Fig. 11.—African Spiny Rat (Lophiomys imhausi).|
The third sub-family is that of the Microtinae, or voles, which are distributed all over Europe, Northern Asia and North America, and are characterized by the tympanic bulla of the skull being filled with honey-combed bony tissue, the small size of the infra-orbital foramen, and the deep pterygoid fossa on the palatal aspect. The humerus lacks a foramen at the lower end; and the molar teeth, as explained and illustrated in the article Vole (q.v.), consist of two longitudinal rows of triangular alternating vertical prisms, and may be either rootless or rooted. Voles, as typified by the water-rat and the tailed field mouse, are stouter built and shorter-nosed rodents than the typical rats and mice, with smaller ears and eyes and shorter tails; all being good burrowers. In the circumpolar Evotomys (represented in England by the red-backed field-mouse) and the nearly allied North American Phenacomys, the molars develop roots in old age; but in Microtus (which includes the water-rat, and is circumpolar) they are rootless throughout life, the genus being one of the largest in the mammalian class (see Vole). Fiber—the muskrats—is a North American aquatic type (see Muskrat), characterized by the compression of the tail. Synaptomys is also North American, and characterized by the grooved upper incisors and the presence of distinct enamel-loops on the outer side of the lower molars. The circumpolar lemmings of the genera Lemmus and Dicrostonyx are noticed in the article Lemming. Ellobius, which many naturalists place in this group, has been mentioned among the Spalacidae.
|Fig. 12.—The Australian Brown-footed Rat (Mus fuscipes).|
The typical rats and mice, together with their nearest relatives, constitute the sub-family Murinae, which is represented by more than three hundred species, distributed over the whole of the Old World except Madagascar. The molars (fig. 13, A) are rooted Fig. 13.—Upper Molars of Mus (A) and Cricetus (B). and have a plate-like structure, with the cusps or tubercles forming three longitudinal rows in those of the upper jaw, but only two distinct ones in the lower. By this structure the Murinae are broadly distinguished both from the Cricetinae (fig. 13, B) and the Microtinae. In the skull the tympanic bulla is hollow, the pterygoid fossa shallow and the zygomatic arch slender, with a rudimentary jugal bone. The tail is long and scaly (fig. 12). The genus Mus, with about a couple of hundred species, includes the true mice and rats (see Mouse and Rat), and has the typical characters of the group, the incisors being narrow and smooth, the molars small, the eyes and ears large and the tip of the muzzle naked. In some cases there may be spines among the fur. None are much larger than the brown rat (M. norvegicus) or smaller than the harvest mouse; and they all have habits generally similar to those of one or other of the English species, although some live in trees like squirrels, or in the water; among the latter being the brown-footed rat (M. fuscipes) of western and southern Australia (fig. 12). The genus Nesocia, is like Mus, but with the incisors and molars broader, and the transverse laminae of the latter more clearly defined. This genus contains a few clumsily built rats spread over Southern Asia from Palestine to Formosa, and from Kashmir to Ceylon (see Bandicoot-Rat). Among other important genera Cricetomys and Eosaccomys (both African) stand apart by the possession of cheek-pouches: C. gambianus being a very large species. The Javan Pithechirus has the thumb opposable, while the Papuan Chiruromys has the tip of the tail naked above and prehensile. The spiny mice, Acomys (or Acanthomys), of Western Asia, Cyprus and Africa, take their name from the fur being almost entirely replaced by flattened spines, and are further distinguished by the rudimentary coronoid process of the lower jaw. Dasymys is an allied African genus; while Arvicanthis includes the African striped mice. Golunda, from India and Africa, is like Mus, but with grooved upper incisors. Vandeleuria, ranging from India to Yunnan, has fiat nails on the first and fifth toes of both feet, and a very long tail; while the Indo-Malay Chiropodomys has a flat nail on the first toe of both feet and a tufted tail. In the Philippines occur the peculiar genera Batomys, Carpomys and Crateromys, confined to the mountains of Luzon, the third remarkable for its huge size and long hair. Mastacomys is like Mus, but with the molars remarkably broadened, and with only four teats. The single species is from Tasmania, though it has been found fossil in New South Wales; it is somewhat similar in size and appearance to the English water-rat, but has longer and softer fur. Uromys differs from Mus in having the scales of the tail not overlapping, but set edge to edge, so as to form a sort of mosaic work. There are several species, spread over the northern part of the Australian region from the Aru Islands to Queensland. Echinothrix is a rat with an extremely elongated muzzle, all the bones of the face being much produced, and the incisors faintly grooved, the only species, E. leucura, being about the size of the common rat, with its fur thickly mixed with spines, a native of Celebes. Australia is the home of the group of jumping species, known as jerboa-rats, characterized by the elongation of the hind limbs, arranged under the genera Notomys, Dipodillus, Ammomys and Conilurus, distinguished from one another by the structure of the molars and the number of teats and foot-pads, the second being further characterized by its long ears.
The large-eared African Otomys and the allied Oreomys (Oreinomys), often made the type of a distinct sub-family, may be included in this section; as well as the small African tree-mice, Dendromys, allied to which is Deomys, peculiar in the circumstance that only the first molar has three rows of cusps, the other two having only a couple of such rows, as in cricetines. Other allied African genera are Steatomys and Lophuromys, which include several species of small mouse-like rodents, with the habits of dormice generally, though some burrow in cornfields. Here also may be noticed the huge Philippine long-haired rats of the genus Phlaeomys, characterized by their broad incisors, transversely laminated molars and large claws. They are often regarded as forming a sub-family by themselves. The gerbils, which are widely distributed over the more or less desert-like regions of the Old World exclusive of the Malay countries and Australia, form the sub-family Gerbillinae. They have long hind limbs, large eyes and ears; and in correlation with the latter an enlarged auditory bulla to the skull, which is hollow and divided into a tympanic and a mastoid portion. The tail is generally long and hairy. There are three pairs of rooted molars, whose crowns carry transverse plates, decreasing in number from three in the first to one in the last tooth. Gerbillus (or Tatera), with a large number of species, has a range coextensive with that of the sub-family; Pachyuromys, with two African species, has a short club-shaped tail and enormous auditory bullae; while the remaining members of the group, which are confined to North Africa, Eastern Europe and Asia, are arranged in the genera Meriones, Psammomys and Rhombomys, the latter represented only by R. opimus from Russia and Central Asia (see Gerbil).
The last representatives of the Muridae are confined to Australasia an the Philippines, and constitute the sub-family Hydromyinae, characterized by the very general presence of only two pairs of molars in each jaw. In the typical Australian and Papuan Hydromys, locally known as water-rats, the molars originally have transverse ridges, the enamel folds between which form cutting edges whose sharpness depends upon the degree to which the teeth have been worn, while the large hind feet are webbed. The typical H. chrysogaster is a large brown rat with an orange belly, which feeds on small fishes and insects. Limnomys, from New Guinea, is a type less specialized for swimming, the hind-feet being much less twisted than in Hydromys, and not so fully webbed. Still less specialized are Chrotomys and Xeromys, which include Philippine land-rats, while Crunomys, from the same area, retains the third molars, and thus connects the group with the Murinae.
Finally, the Philippine Rhynchomys is represented by a rat with two pairs of molars and a long shrew-like nose, the zygomatic arch of the skull being also placed unusually far backward.
Strand-Moles.—With the so-called strand-moles of South Africa, forming the section Bathyergoidea, and the family Bathyergidae, which were formerly placed with the Spalacidae, we come to the first of two sections in which the lower jaw has a totally different form to that obtaining in all the preceding groups. In the rodents now to be considered, the angular process of the lower jaw arises from the outer side of the sheath of the incisor. The malleus and incus of the internal ear are united, and there is no transverse canal in the skull. At least one pair of premolars is present in each jaw; and these teeth and the molars typically have one outer and one inner enamel fold. There is no foramen at the lower end of the humerus, and no horny layer in the stomach.
In the Bathyergoidea the scaphoid and lunar of the carpus are separate, the tibia and fibula united and the clavicles normal. The masseter muscle does not pass through the narrow infra-orbital canal, and the temporal muscle is large. All the Bathyergidae are African, and adapted to a burrowing life, having minute ears and eyes, a short tail and the thumb armed with a large claw. The largest species represents the genus Bathyergus, while several smaller kinds are included in Georychus. The former constructs its tunnels in the sandy flats near the shore at the Cape, but the latter generally frequent higher ground. In both genera there is only a single pair of premolars in each jaw, but in the smaller Myoscalops there are usually three pairs of these teeth. The most remarkable members of the family are the sand-rats of Somaliland and Shoa, forming the genera Heterocephalus and Fornarina, in which the premolars may be reduced to two pairs. They have large heads, projecting incisors, no ears, almost functionless eyes and moderately long tails; the skin, with the exception of a few hairs on the body and fringes on the feet, being naked. They spend their whole time buried in the hot desert sand, in which they construct burrows, throwing up at intervals small hillocks.
Fig. 14.—Skull of the Capybara (Hydrochaerus capybara), reduced.
Porcupines.—In the second section, or Hystricoidea, including several families, the skull (fig. 14) is characterized by the heavy zygomatic arch, the middle portion of which is formed by the more or less straight and horizontal jugal, and the large infra-orbital canal, traversed by a portion of the masseter muscle. The tibia and fibula are separate, but the scaphoid and lunar are united, and the clavicles are generally incomplete. There is never more than one pair of premolars, and the original ridges of all the cheek-teeth have become obscured and complicated by the development of secondary enamel-folds. The majority of these rodents, many of which are of large size, are terrestrial, but a few are burrowing, others arboreal and two or three aquatic.
The Old World porcupines, constituting the family Hystricidae, are terrestrial, stoutly built rodents, with limbs of subequal length in front and behind, and the skin covered with strong spines. The upper lip is cleft, the jugal lacks an inferior angle, the fore part of the skull is short and broad; the cheek-teeth are partially rooted, with external and internal enamel-folds, the soles of the feet are smooth, there are six pairs of teats, the clavicles are imperfect and the tail is not prehensile. In the typical genus Hystrix, which is represented in all the three great continents of the Old World, and extends as far east as Flores and Celebes, the skull is swollen and convex, the spines are cylindrical, and the tail is short and covered with spines and slender-stalked open quills. In Atherura fasciculata of the Malay Peninsula the spines are flattened, and the tails long and scaly, with a tuft of compressed bristles. A closely allied species, A. africana, inhabits Western Africa. The third genus is Trichys (see Porcupine).
Fig. 15.—The Brazilian Tree-Porcupine (Synetheres (or Cöendu) prehensilis).
American Porcupines.—All the New World porcupines, representing the family Erethizontidae (or Cöendidae) are arboreal in their habits, and have the upper lip undivided, the cheek-teeth rooted, the clavicles complete, the soles of the feet tuberculated and three pairs of teats. Erethizon dorsatus, the urson, is distributed all over the forest regions of North America; Synetheres (or Cöendu) prehensilis, the prehensile-tailed porcupine of South America (fig. 15), represents a genus in which the whole upper surface of the body is protected by long white-tipped spines; Chaetomys subspinosus is clothed with strong wavy bristles. In the last two genera the feet have four toes, in place of the five of Erethizon (see Porcupine).
Cavy Group.—In the family Caviidae, typified by the cavies (or guinea-pigs), may be included a large number of South and Central American rodents, among which the agoutis and pacas are often ranked as a family (Dasyproctidae) by themselves. The Caviidae, in the present more comprehensive sense, include the giants of the rodent order. Many of them, like ungulates, are specialized for swift running, and have unusually long limbs, with ridges developed on the articular surfaces of the lower bones; the clavicles are more or less reduced; the thorax is more compressed than usual, with a narrower breast-bone; and there is a marked tendency to the reduction or loss of the lateral toes, more especially in the hind limb. Since these rodents walk more or less entirely on their toes, in such a manner that the edges of the claws or nails come in contact with the ground, these tend to assume somewhat of a hoof-like character; while the foot-pads are more or less horny. The tail is generally very short, and its basal vertebrae are often fused with the sacrum. In the skull the lachrymal bone is large, the par-occipital process is directed vertically downwards and the tympanic bulla is hollow. In the soft parts the caecum is very large, the penis is armed with a pair of barbed horny claspers and the scrotum is spiny.
Special interest attaches to the most aberrant member of the family, the Peruvian Dinomys, known for more than thirty years only by a single specimen taken in a house in Lima, and only lately rediscovered. It is a large rodent known to the Tupi Indians as the paca-rana, or false paca, in allusion to the resemblance of its coloration to that of the true paca, from which it differs by its well-developed tail, the absence of cheek-pouches, the full development of all live toes and the wider thorax. The Tupi name may be adopted as the popular title of the species. Dr E. Goeldi states that the paca-rana is a rodent of phlegmatic and gentle disposition, which may account, perhaps, for its rarity, if, indeed, it be really scarce in its native home, which is probably the eastern slopes and tablelands of the Bolivian and Peruvian foot-hills bordering on Brazil, inclusive of the headwaters of the Purus, Acre and Jurua rivers. In the true pacas, Coelogenys (or Agouti), the first front toe is small, and both the first and fifth digits of the hind-foot are much inferior in size to the other three. The most remarkable feature of the genus is, however, the extraordinary development of the zygomatic arches of the skull, which are enormously expanded vertically, forming great convex bony capsules on the sides of the face, enclosing on each side a large cavity lined with mucous membrane internally, and communicating by a small opening with the mouth. C. paca is a white-spotted rodent, about 2 ft. long, and lives generally in the forests or along the banks of rivers (see Paca). The Agoutis, Dasyprocta, include several species of slender-limbed rodents, with three hind-toes, inhabiting Central and South America, one (D. cristata) extending into the West Indian islands. The members of both Coelogenys and Dasyprocta are terrestrial in their habits, and have the fore- and hind-limbs subequal, hoof-like claws, short or obsolete tail and rudimentary clavicles. The masseteric ridge of the lower jaw is obsolete, the palate broad, the incisors long and the molars semi-rooted, with external and internal enamel-folds (see Agouti). The remaining and more typical members of the family, one of which is aquatic, are characterized by their short incisors, the strong masseteric ridges on the sides of the lower jaw, the long and curved par-occipital and the palate contracted in front. Fore-feet with four digits, hind-feet with three; clavicles imperfect; molars divided by enamel-folds into transverse lobes; milk-teeth shed before birth. In the true cavies, or couies, Cavia, the fore- and hind-limbs are short and of subequal length, the ears are short and there is no tail. They include several species widely distributed throughout South America, extending even to the straits of Magellan, from one of which (C. cutleri of Peru) the guinea-pig is derived. The maras (Dolichotis) have the limbs and ears long and the tail very short. D. patagonica is a large species, nearly 3 ft. long, inhabiting the gravelly plains of Patagonia, while D. salinicola is a much smaller rodent from the salt-lagunas of Argentina. The palate is so much contracted in front that the premolars of opposite sides touch by their antero-internal edges. Hydrochaerus, in which all the feet are fully webbed, includes a single species, the capybara, or carpincho, the largest of living rodents. The skull (fig. 14) is distinguished not only by its great size, but by the enormous development of the par-occipital processes and the complex structure and large size of the last molars (see Cavy and Capybara).
Chinchilla Group.—The family, Chinchillidae, typified by the well-known chinchilla, includes a small number of South American rodents with large ears and proportionately great auditory bullae in the skull, elongated hind-limbs, bushy tails, very soft fur and perfect clavicles. The jugal is without an inferior angle, and extends forwards to the lachrymal; the palate is contracted in front and deeply emarginate behind; the incisors are short, and the molars divided by continuous folds into transverse plates; and the two halves of the lower jaw are welded together in front. It includes three existing genera, represented by some five species. Of these the true chinchilla, Chinchilla lanigera, C. brevicaudata, Lagidium peruanum and L. pallipes, are restricted to the alpine zones of the Andes from the northern boundary of Peru to the southern parts of Chili; while Lagostomus trichodactylus (or Viscaccia viscaccia), the viscacha, is confined to the pampas from the Uruguay river to the Rio Negro. In Chinchilla the fore-feet have five and the hind four digits, the tail is long and bushy, and the auditory bullae are enormous, appearing on the top of the skull; Lagidium has four digits in both fore- and hind-feet, and Lagostomus three only in the hind-feet, while the auditory bullae are much smaller (see Chinchilla and Viscacha).
Hutia Group.—The three remaining families of the Hystricoidea, of which one is African while the other two are chiefly South American, are very closely allied and often brigaded in a single family group. In the Capromyidae, which includes only the South American and West Indian hutias, the South American coypu and the African cane-rats, the tympanic bulla of the skull is hollow, the par-occipital process straight, the lachrymal small, and the cheek-teeth rooted, with deep enamel-folds; the first front toe being occasionally absent. Of the few living representatives of the group, the genus Myocastor (or Myopotamus) is represented only by the South American coypu, M. coypu, which is aquatic in its habits, and measures about 2 ft. in length, being the largest member of the group. It has a long tail, brown fur and red incisors, and lives in burrows near water, feeding on aquatic plants. The hutia (Capromys pilorides) is nearly as large, arboreal in habits, and a native of Cuba, where it is the largest indigenous mammal. Other species occur in Cuba, Jamaica and the Bahamas, while a Venezuelan species, Procapromys geayi, represents a separate genus. In one kind the tail is prehensile. All these rodents are remarkable for the manner in which the liver is divided into minute lobules. Plagiodontia aedium, another member of the group, is peculiar to Hayti. The African cane-rats, Thryonomys (or Aulacodus), are large terrestrial rodents, ranging from the centre of the continent to the Cape, easily recognized by their deeply fluted incisors (see Coypu). The Octodontidae, which are exclusively South American, differ from the preceding family by the tympanic bulla being filled with cellular bony tissue, and by the par-occipital process curving beneath it, while the cheek-teeth are almost or completely rootless and composed of parallel plates. The first front toe may be absent. The more typical members of the family are rat-like burrowing rodents, living in communities. The typical genus is represented by the degu (Octodon degus) and several nearly related species; other genera being Ctenomys, Octodontomys (Neoctodon), Aconaemys, Spalacopus and Abrocoma; the latter taking its name from its unusually soft fur. Among these, the tuco-tucos (Ctenomys) are characterized by their burrowing habits, almost rudimentary ears, small eyes, short tails and the kidney shaped grinding-surfaces of their cheek-teeth. They take their name of tuco-tuco from their cry, which resembles the blows of a hammer on an anvil, and may be heard all day as the little rodents move in their burrows, generally formed in sandy soil. In some districts the ground is undermined by these burrows, in which stores of food are accumulated. The species of Octodon have larger ears, longer, tufted tails and the sides of the cheek-teeth indented by plates of enamel; they are chiefly found in hedgerows and bushes, where they burrow. In Abrocoma the tail has no tuft, the ears are still larger and the lower cheek-teeth more complex than the upper ones. Aconaemys is an allied Chilean genus in which the enamel-folds meet across the molars. Several of these rodents live in the Andes, where the ground is covered for months with snow. The second group of the family is formed by the genera Loncheres, Dactylomys, Echi[no]mys, Proechimys and a few others, the members of which are rat-like rodents, with long scaly or furry tails, and frequently flattened spines mingled with the fur of the back. Most species are brown above and whitish beneath, but in some the lighter tints extend on to the sides, shoulders and head, communicating a coloration somewhat like that of a guinea-pig (see Octodon). The North African gundis (Ctenodactylus gundi and Ct. vali) are the types of an African family, which also includes the genera Massoutiera, Peclinator and Petromys. In the gundi the two inner toes of the hind-foot are furnished with a horny comb and bristles for the purpose of cleaning the fur, and the tail is very short; but in Pectinator the tail is longer. Petromys has a still longer and more bushy tail, and no comb to the hind-feet. The gundi is a diurnal species, inhabiting rocky districts, and having habits very similar to those of a jerboa. Of these Ctenodactylus and Pectinator are characterized by the union of the incus and malleus of the internal ear, the free fibula and the almost rootless cheek-teeth. The premolar is very small, thus showing an approximation to the Myoidea, although in other respects Petromys appears to approximate to the Hystricidae.
Picas and Hares.—The remaining rodents, which include two families—the picas (Ochotonidae) and the hares and rabbits (Leporidae)—constitute a second sub-order, the Duplicidentata, differing from all the foregoing groups in possessing two pairs of incisors in the upper jaw (of which the second is small, and placed directly behind the large first pair), the enamel of which extends round to their posterior surfaces. At birth there are three pairs of incisors, but the outer one is soon lost. The incisive foramina are large and usually confluent; the bony palate is very narrow from before backwards; there is no alisphenoid canal; the fibula is welded to the tibia, and articulates with the calcaneum; and the testes are permanently external. All are terrestrial, and in many cases burrowing, in their habits, and some of them are of extreme fleetness. The Ochotonidae are represented at the present day only by the single genus Ochotona (Lagomys), which includes all the picas, or mouse-hares. They are small rodents with complete clavicles, fore- and hind-limbs of nearly equal length, no external tails and short ears. Skull depressed, frontals contracted and without post-orbital processes; p. 1 or 2; molars rootless, with transverse enamel-folds. In some cases the molar-formula is 2. The genus includes about a score of species of guinea-pig-like animals, inhabiting chiefly the mountainous parts of Northern Asia (from 11,000 to 14,000 ft.), one species only being known from South-east Europe and several from the Rocky Mountains and Alaska.
From the picas the hares and rabbits (Leporidae) are distinguished by the imperfect clavicles, the more or less elongated hind-limbs, short recurved tail (absent in one case) and generally long ears. The skull is compressed, with large wing-shaped post-orbital processes (fig. 16); p. 3. With the exception of Australasia, the family has a cosmopolitan distribution; and its numerous species resemble one another more or less closely in general external characters. In all the fore-limbs have five and the hind four digits; and the soles of the feet are densely clothed with hairs Fig. 16.—Skull of the Common Hare (Lepus europaeus). similar to those covering the legs; the inner surface of the, cheeks being hairy. Although the family has such a wide distribution, the greater number of the species are restricted to Europe, northern and central Asia and North America; South America having very few. Till within the last few years the majority of naturalists followed the practice of including all the members of the family in the genus Lepus. It is true that Mr E. Blyth long ago proposed the name Caprolagus for the remarkable spiny rabbit of the western Himalayas, while the generic name Oryctolagus was suggested later for the rabbit, and Sylvilagus for the American “cotton-tails”; but none of these was accorded general acceptation. Of late years, however, zoologists have come to the conclusion that generic subdivisions of the Leporidae are advisable. In 1899 Dr Forsyth Major proposed a classification of the family in which a number of species were grouped with the spiny rabbit in the genus Caprolagus, whilst Oryctolagus was taken to include not only the common rabbit, but likewise the Cape hare. A more recent classification is that of Mr M. W. Lyon, in which by far the largest number of species of the family are retained in the original genus Lepus, which has also the widest geographical distribution of all the genera. It is typified by the blue hare (Lepus timidus), next to which comes the common hare (L. europaeus) and certain other allied forms. The jackass-hares of Mexico, &c., such as L. californicus, form a second sub-group; while these are in turn followed by the American hare (L. americanus) and its immediate relatives. The cotton-tails, or wood-rabbits, of North and South America are regarded as forming a genus, Sylvilagus, by themselves, which includes the Brazilian and Paraguay hares, and appears to be chiefly distinguished by a certain feature in the parietal region of the skull. Under the name of Oryctolagus cuniculus, the rabbit is considered to represent a genus by itself, specially characterized by the shortness of the ears and hind-feet. The swamp-rabbit (L. palustris) and water-hare (L. aquaticus) of the southern United States form the group Limnotragus, characterized by the harsher fur, the shorter ears, tail and hind-feet, and the complete fusion of the post-orbital process (which is so distinct in the typical hares) with the adjacent parts of the skull, so that neither notches nor perforations are developed in this region. The short-tailed rabbit of the western United States (Brachylagus idahoensis) is the sole member of a group allied in general characters to the typical Lepus, but distinguished by the unusually short tail. Another group is Pronolagus, typified by the Cape thick-tailed hare, the so-called Lepus crassicaudatus, which is externally similar to Lepus proper, but has the skull and teeth of the general type of the next group. The tailless rabbit of Mount Popocatepetl, Mexico, originally described as a distinct generic type, under the name of Romerolagus nelsoni, is broadly distinguished by the entire absence of the tail, and the short ears and hind-feet, its general form being like that of the Liu-Kiu rabbit, while, as in the latter, the post-orbital process of the skull is small, and represented only by the hinder half. Next come three remarkable rabbits from the Indo-Malay countries, all closely allied, although regarded as representing three generic groups, Nesolagus, Caprolagus and Pentalagus. In all three the skull is of the type of Romerolagus. The first is represented by the Sumatran rabbit, the so-called N. netscheri, which apparently differs from the spiny rabbit mainly by the pattern of the cheek-teeth. The spiny rabbit, separated from Lepus by Blyth in 1845 under the name of Caprolagus hispidus, is an inhabitant of Assam and the adjacent districts, and distinguished by its harsh, bristly fur and short ears and tail. In the Liu-Kiu rabbit (Pentalagus furnessi) the coat is equally harsh, but the ears and hind-feet are shorter, and there are only five (in place of the usual six) pairs of upper cheek-teeth. In the loss of the last upper molar, the Liu-Kiu rabbit approximates to the picas, as does the tailless rabbit in the abortion of its caudal appendage. Mr Lyon's scheme seems to be the best attempt to explain the affinities of the members of the group. Whether all his genera be adopted, or all the species be included in Lepus, must largely be a matter of individual opinion. If the latter course be followed, Mr Lyon's genera must be reduced to the rank of sub-genera, and his sub-generic divisions of Lepus and Sylvilagus ignored. (See Hare and Rabbit.)
Among extinct rodents, only a few of the more important types may be noticed. As to the origin of the order, we are still to a great extent in the dark; and even the relations of the Duplicidentata to the Simplicidentata are not yet fully understood. With regard to the latter point, it is, however, considered probable that both are branches of a common stock, which diverged from each other before all the typical rodent characters were acquired. As to the ancestral stock of the order, it has been suggested that this is represented by certain Lower Eocene European and North American mammals, at one time regarded as primitive Primates. In Europe these include Plesiadapis and Protoadapis, and in North America Mixodectes, Microsyops and Cynodontomys; the last three constituting the family Mixodectidae. Possibly the European forms, in which the dental formula has been given as i. 3, c. 0, p 2, m. 3, and there is a gap between the incisors and the cheek-teeth, are more nearly related to modern rodents than the American types, and may indeed belong to the same order. On the other hand, the American forms, which have one pair of large chisel-like incisors in the lower jaw, also possess a lower canine, and show no marked gap in front of the cheek-teeth, nor any indication of the characteristic rodent backwards movement of the lower jaw. On these grounds, while admitting that they are allied to the rodents, it has been pointed out that they can scarcely be included in the Rodentia, and the order Proglires has in consequence been proposed for their reception.
Whatever may be the true affinity of these problematical mammals, undoubted rodents are known from the Lower Eocene of both Europe and North America. In Europe these form the genus Ischyromys and the family Ischyromyidae, and have premolars 2, and all the cheek-teeth low-crowned, with simple cusps or ridges. Possibly they are akin to the Sciuridae. In America, Paramys, with transversely ridged molars, is allied; and the European Sciuromys should perhaps find a place in the same neighbourhood. A more advanced phase is represented in the European Lower Oligocene by the Pseudosciuridae, with the genera Pseudosciurus, Sciuroides, Trechomys, Theridomys, &c., in which part of the masseter passes through the broad infra-orbital canal, and the premolars are 1; the molars being low-crowned, many-rooted and either cusped or ridged. These rodents are thought to be allied to the Anomaluridae; and it is partly on their evidence that the family Pedetidae is placed next the latter. Here it may be mentioned that Leithia, from the Pleistocene of Malta, originally regarded as a giant dormouse, seems near akin to Anomalurus. In the highly specialized mastoid region of the skull, the North American Oligocene Protoptychus approaches to Dipopodomys, while the contemporary Gymnoptychus and Entoptychus likewise appear referable to the Geomyidae. The Upper Oligocene Cricetodon in Europe and Eumys in America are the earliest known forerunners of the cricetine Muridae; while at the same time primitive beavers appear in the form of Steneofiber, to be succeeded in the European Pleistocene by the gigantic Trogontherium.
The still larger North American Pleistocene Castoroides, known by one species of the size of a bear, and the allied West Indian Amblyrhiza, appear to be specialized beavers, although they have been referred to a family by themselves. Near akin is the North American Miocene family Mylagaulidae, typified by Mylaganlus, but including Mesogaulus and Protogaulus. Although showing some dental characters approximating to the porcupines, these rodents are regarded as allied to the Castoridae, although forming an isolated type. The prominent feature, writes Mr E. S. Riggs, is the unusual development of the premolar to the exclusion of the posterior teeth. Associated with this is the strength and sharpness of the lower jaw, the prominence and anterior position of the masseteric ridge, and the depth of the ramus from the alveolar line to the angle. These indicate unusual capacity for crushing or grinding; while the last premolar is a crushing implement, which has reached the highest degree of specialization known in Rodentia. It is suggested that these teeth may have been employed for cracking nuts or hard seeds, although also used for grinding. The remarkable North American Ceratogaulus, with a large bony nasal horn, belongs to the same family. To discuss the remaining Miocene and later fossil Simplicidentata would be doing little more than adding to the generic names referable to the various existing families. It may be mentioned, however, that the distribution of these later Tertiary types accords very closely with that of their existing relatives; the families of South American hystricoids being represented by a number of extinct genera in the formations of Argentina and Brazil. Special mention may be made of Megamys, from the caves of Brazil, which, while apparently allied to the living viscacha, attained dimensions approximating to those of a hippopotamus.
As regards the Duplicidentata, it appears that the families Ochotonidae and Leporidae had become differentiated as early as the Lower Miocene. Titanomys is the earliest form, from the Middle Miocene, succeeded by Lagopsis, and then by the modern Ochotona. In this line there is a tendency to lose the last upper molar, but in Prolagus, which ranges in the Pliocene from Sardinia and Corsica to Spain, and forms a side-branch, the corresponding lower tooth has likewise disappeared. In contradistinction to Titanomys, in which the cheek-teeth are rooted, is the North American Upper Oligocene Palaeolagus, where they are rootless. In general dental characters, especially the retention of three pairs of molars, this genus approximates to the Leporidae, although in the absence of post-orbital processes and the pattern of the molars it departs less widely from the modern Ochotonidae than does Prolagus.
Authorities.—The above article is partly based on that by G. E. Dobson in the 9th edition of this work. See also H. Winge, Jord Fundene og Nulevende Gnadere (Rodentia), E. Museo Lundi (1888); C. J. Forsyth-Major, “On some Miocene Squirrels, with Remarks on the Dentition and Classification of the Sciuridae,” Proc. Zool. Soc. London (1893); “On Fossil and Recent Lagomorpha,” Trans. Linnean Soc. London, vol. vii. (1899); T. S. Palmer, “A List of the Generic and Family Names of Rodents,” Proc. Zool. Soc. Washington, vol. xi. (1897); O. Thomas, “On the Genera of Rodents,” Proc. Zool. Soc. London (1896); T. Tuhlberg, Über das System der Nagethiere (Upsala, 1899); H. F. Osborn, “American Eocene Primates, and the Supposed Rodent Family Mixodectidae,” Bull. Amer. Mus. Nat. Hist. vol. xvi. (1902); W. Lyon, “Classihcation of the Hares and their Allies,” Smithsonian Miscell. Collections, vol. xlv. (1903). Also numerous papers by O. Thomas, in Proc. Zool. Soc. London and Annals and Magazine of Nat. Hist., and by several American naturalists in transatlantic zoological serials. (R. L.*)