Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/441

Rh 110DE5TIA.J MAMMALIA 419 Hydromys, with two species from Australia, Tasmania, and New Guinea, includes the AVater-llats of the Australian region, with partially webbed toes. They are distinguished from all other Kodents by the small number of their molars. Subfamily 3. Platacanthomyinae. M |, with transverse laminse ; fur mixed with ilattened spines ; tail densely hairy. Platacanthomys, with P. lasiiirus, a small Dormouse-like species from western India, inhabiting the rocky mountains of Trav ancorc. Subfamily 4. Grerbillinae. Incisors narrow, molars with trans verse lamina; ; auditory bullre ossese usually large ; hind limbs elongated ; tail generally long and hairy. Palcearctic, Indian, and Ethiopian. Gcrbillus, with nearly fifty species, has a range coextensive with that of the subfamily. Pachyuromys is distinguished by the enormous size of the auditory bullae, as well as by the short fleshy club-shaped tail. Mystromys, Otomys, and Dasymys differ in the form of the molars, and are represented by a .few species, all from South Africa. Subfamily 5. PJ.la3omyinae. Incisors broad, molars with transverse laminae ; claws large. Indian. Phlseomys, with -P. cumingii from the Philippines, and JVcsokia, with eight species widely distributed throughout the Indian Region. The latter (distinguished from the former genus by the short, subnaked, scaly tail) includes the Great Bandicoot Eat of India (A r . landicota). Subfamily 6. Dendromyinas. Incisors convex in front, molars tuberculate ; ears hairy ; claws long. Ethiopian. Three genera, Dcndromys, Stcatomys, and Lophurbmys, include several species of small Mouse-like Kodents with the habits of Dormice generally, though some burrow in corn-fields. Subfamily 7. Cricetinae. Molars tuberculate. Large internal cheek-pouches. Palsearctic and Ethiopian. Of the three genera, Cricetus, Saccostomus, and Cricdomys, the last is distinguished from the others by the grooved upper incisors, while Sttccostomus is separated from Cricetus by the tubercles of the molar teeth being arranged in threes. The cheek pouches in Cricetus are very large, and their walls are connected with muscles arising from the lumbar vertebrae. The best-known species is C. frumentarius. See HAMSTER. Subfamily 8. Murinae. Molars tuberculate, at least in youth. Cheek-pouches absent. Tail scaly, more or less naked. Cosmo politan. This includes the typical murine forms, divided into fifteen genera with over three hundred species, of which nearly half, however, are contained in the genus Mus. M. decumanus, the common Grey Eat, and M. musculus (see MOUSE) are familiar examples which have been introduced in ships into almost every part of the habitable world. In Acomys the skull and teeth are as in Mus, but the fur is mixed with sharp flattened spines. A. dimidiatus presents the appearance of a little Hedgehog when its spines are erected ; it inhabits the stony deserts of Arabia Petrrea and Palestine, and feeds on bulbs. Echiothrix, Uromy.i, and Hapalotis, the latter with about a dozen species, are limited to the Australian region. Bracliytarsomys, Ncsomys, Hallomys, and Hypogcomys, each with one or two species, are peculiar to Madagascar, where they alone represent the order. Drymomys, Holochcilus, Oclictodon, and ffesperomys are New-World genera, tin last-named including many species representing the Old-World Mice, but distinguished by the indenting enamel folds of the molars. Of the remaining genera, Reithrodon, with grooved incisors, includes two very remarkable Rabbit-like species, one inhabiting Patagonia, the other Tierra del Fuego. II. Molars Semi-Rooted or Rootless. Subfamily 9. Arvicolmae. Molars composed of triangular prisms placed alternately ; limbs moderate ; tail moderate or short, hairy. Palsearctic and Nearctic. Arvicola, with over thirty species, includes the Voles, of which the Field-Vole (A. agrcstis) and the Water-Vole (A. amphibius) are well-known examples (see VOLE). Myodes, distinguished by the hairy foot-sole, includes two species, of which M. Icmmus, of the Scandinavian Peninsula, is remarkable for its extraordinary migrations (see LEMMING), fiber is represented by a single large species, F. zibcthicus (see fig. 94), the Musk-Rat, or Musquash, a Heaver-like Water-Rat with webbed toes, but a laterally flattened tail, inhabiting the banks of rivers and lakes in North America, and constructing dome-roofed dwellings like those of the Beaver ; it is much hunted for its fur. Subfamily 10. Siphneinae. Molars as in Arvicolinse ; form cylindrical ; ear-conch rudimentary ; limbs and tail very short. Palaaarctic. Ellobius, with short claws, connects the species of this subfamily of fossorial Mole-like Rodents with the Arvicolinse. Siphneus, on the other hand, leads to the next family, which includes the true Mole-Rats ; the species, which chiefly inhabit northern Asia, closely resemble the Golden Moles (Chrysochloris) in general form and in the great development of the claws of the fore feet (compare fig. 96 with fig. 64, p. 405). Family 4. SPALACIBJE. Rodent Moles, with very small or rudimentary eyes and c-ar- conchs, large claws, and short or rudimentary tail. Form cylindrical. Incisors large; molars rooted, with re-entering enamel folds ; palate narrow. Subfamily 1. Spalacinae. Angular part of the mandible arising from the lower edge of the socket of the lower incisor. Palaearctic, Indian, and Ethiopian. Spalax, with S. typhlus of south-east Europe, agrees with the insectivorous Golden Moles in the complete external absence of tlie eye, which is covered by the hairy skin, showing similar adaptive FIG. 9G. Siphneus armandii (reduced). A. Milne-Edwards, Mammif. Tibet. modification in widely removed species. In Rhizomys, including several species from China, Tibet, Malay Peninsula, and eastern Africa, the eye is very small. Subfamily 2. Bathyerginae. Angular part of the mandible arising from the side of the socket of the lower incisor. Ethiopian. Bathyergus, with B. ynaritimus, the Great Rodent-Mole, inhabit ing the sand-dunes along the coast in the vicinity of the Cape of Good Hope, is distinguished chiefly by its grooved incisors from the other species included in the genera Gcorychus and Heliophobius, the former with several, the latter with one species, and differing from both in the presence of two or three premolars. Family 5. GEOMYID.E. Terrestrial or fossorial Rodents, with large cheek-pouches opening on the cheeks outside the mouth. The squamosal bones are much expanded, and the malars extend forwards to the lachrymals. Pm {- ; molars rooted or rootless. Nearctic and Neotropical. Subfamily 1. Geomyinae. Incisors broad ; mastoid not appear ing on the top of the skull ; eyes small ; ear-conch rudimentary ; limbs short, subequal. Fossorial. Gcomys bursarius, the common Pouched Rat of North America, with deeply-grooved incisors, inhabits the plains of the Mississippi, living in burrows like the Mole. Four other species from the Southern States, Mexico, and Central America are recognized. Thomomys talpoides, with plain incisors, extends all over Canada and North America west of the Rocky Mountains. Subfamily 2. Heteromyinae. Incisors narrow; mastoid appear ing largely on the top of the skull ; eyes and ears moderate or large ; hind limbs and tail elongated. Terrestrial. Dipodomys has the molars rootless ; D. phillipsi is the Kangaroo- Rat of the desert regions east of the Rocky Mountains. Pcrognathus and Ilcteromys have rooted molars ; the latter genus is distinguished by the presence of flattened spines among the fur, with species extending into South America. Family 6. DIPODID^E. Terrestrial leaping Rodents, of slender form, with elongated hind limbs. The incisors are compressed, the molars have transverse enamel folds, the infra-orbital opening is rounded and very large, the malar ascends in front to the lachrymal in a flattened per pendicular plate (see fig. 89, p. 415), and the mastoid part of the auditory bulla is usually greatly developed. Subfamily 1. Zapodinee. Molars rooted ; cervical vertebra? free ; metatarsals separate ; hind feet with five digits. Nearctic. Zapus hudsonius, the American Jumping-Mouse, extends over almost the whole North-American continent from Labrador to Mexico. Subfamily 2. Dipodinse. Molars rooted; cervical _ vertebras more or less ankylosed ; metatarsals united ; hind feet with three functional digits only. Palsearctic and Ethiopian. This includes the true Jerboas. It contains three genera :