Page:EB1911 - Volume 23.djvu/464

Rh 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 .)

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.

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 ); 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.

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 (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 ). Fiber—the muskrats—is a North American aquatic type (see ), 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. Ellobius, which many naturalists place in this group, has been mentioned among the Spalacidae.