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 flat 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
