Page:EB1911 - Volume 23.djvu/463

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 * [[Image:EB1911 Rodentia - Red Scaly-tailed Squirrel.jpg|325px|center]]
 * align="center"| 7.—Red Scaly-tailed Squirrel (Anomalurus fulgens).
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 * align="center"| 7.—Red Scaly-tailed Squirrel (Anomalurus fulgens).
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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 .)

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 .)

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 .)

Mole-Rats.—The mole-rats (Spalacidae) bring us to the mouse-like section, or Myoidea, in which there are no premolars and the 9.—Skull of the Muskrat (Fiber zibethicus). Natural size. molars may be occasionally reduced to ; 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.