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.
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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).
