418 M A M M A L I A [EODENTIA. Family 2. Arboreal or terrestrial Rodents, with cylindrical hairy tails, without scales, and with twelve or thirteen pairs of ribs. Skull with distinct post-orbital processes ; infra-orbital opening small ; palate broad ; pm % ; first upper premolar very small or deciduous ; molars rooted, tubercular. Subfamily 1. Sciurinee. Incisors compressed, form slender, tail long and hairy (True Squirrels). Cosmopolitan (excluding Australian region). There are four genera. Ftcromys, limbs united by a cutaneous expansion forming a parachute, the supporting cartilage of which springs from the carpus, includes the Flying Squirrels of both hemispheres. Sciurus, with more than eighty species, comprises the true Tree Squirrels. To this group also belong the Short-eared Squirrels of the genera Xerus and Tamias ; the first-named, distinguished from Sciurus by possessing only two pairs of mam mary teats, by the comparatively short tail, and by the fur being mixed with flattened spines, includes a few African species which live in burrows ; Tamias, separated by the presence of large internal cheek-pouches, includes the well-known Ground Squirrels of North America, of which one species (T. asiat-icus) extends into North Europe and Asia. See SQUIRREL. Subfamily 2. Arctomyinse. Incisors not compressed ; form stout ; tail short (Marmots). Palsearctic and Nearctic. Spermophilus, distinguished by its large cheek-pouches and by the absence or rudimentary condition of the pollex claw, resembles Tamias in the slender form of the body, and connects the Marmots with the true Squirrels. The distribution and habits of the species are similar to those of Tamias. Cynomys, with shallow cheek- pouches, long pollex claw and stout form, includes the well-known Prairie Dogs peculiar to North America, which live together in large communities, inhabiting burrows which they excavate at short distances apart ; they feed on the buffalo-grass which covers the plains. The small burrowing owl (Athene cunicularia) and the rattlesnake are often found inhabiting their burrows, the former probably availing itself of the convenience of a ready-made habi tation, the latter coming there to feed on the young Marmots. Arctomys, distinguished by its rudimentary pollex, includes the true Marmots. See MARMOT. Family 3. Terrestrial Rodents, distinguished from Sciuridse by the absence of post-orbital processes, the depressed form of the skull, and the rootless molars. Premolars f, the first upper one small. Haplodon, with H. rufus, from North America west of the Rocky Mountains, alone represents the family. The habits of the single species are similar to those of the Prairie Dog. Family 4. CASTORID.S;. Natatorial Rodents, with massive skulls, without post-orbital processes, with mandibular angle rounded, and with semi-rooted or rootless molars with re-entering enamel folds ; pm -. Castor, with one species only, C. fiber, the Beaver of the northern parts of Asia, Europe, and America. 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 cesophageal orifice ; the anal and urethro-genital orifices open within a common cloaca ; the tail is broad, horizontally flattened, and naked ; and the hind feet are webbed. See BEAVER. Section II. MTOMORPHA. Zygomatic arch slender, the malar rarely extending far forwards, and usually supported below by the long zygomatic process of the FIG. 94. Skull of Fiber zibethicus. Natural size. maxillary ; no post-orbital processes ; infra-orbital opening vari able ; mandible with the angular part arising from the inferior surface of the bony socket of the lower incisor (except in BathyergineB); clavicles well-developed (except inLophiomys); tibia and fibula united. Family 1. MYOXID^E. Arboreal Rodents, with long hairy tails, large eyes and cars, and short fore limbs. The intestine is without a caecum ; the skull has. contracted frontals ; the infra- orbital foramen is moderate, high, and narrow ; and the mandible has a long and slender coronoid process. Pm $ ; molars rooted, with transverse enamel plates. Palsearctic and Ethiopian. There are four genera : l[yoxus, with M. glis of Europe, with bushy, distichous tail, simple stomach, and large molars with well- marked enamel folds ; Muscardinus, with M. avcllanarius, the common Dormouse, distinguished by the cylindrical bushy tail am? thickened glandular walls of the cardiac extremity of the oesophagus : Eliomys, with about six species, with tufted and distichous tail., simple stomach and smaller molar teeth with concave crowns and faintly marked enamel folds ; and Graphiurvs, with two species, with short cylindrical tail ending in a pencil of hairs, and very small molars almost without trace of enamel folds. The Dormice form a very natural family, distinguished from all other Rodents, by the absence of a caecum. In their habits and form they evidently closely approach the Squirrels. Family 2. LoPHIOMTIDJE. Arboreal Rodents, with rudimentary clavicles and rooted tuberculate molars. Premolars none. Skull murine in form, but the temporal fossae are completely arched over by thin plates arising from the temporal ridge and malar bone. Intestine with a smal! FIG. V5.Lophiomys imhausi (reduced). A. Milne-Edwards. Lophiomys, with L. imhausi (fig. 95), alone represents the family. This very extraordinary species from north-east Africa differs from all other Rodents in the peculiar granulated plates which cover the temporal fossae, and from all the species of the section in the rudimentary condition of the clavicles as well as in the possession of an opposable hallux. The hair is very peculiar in structure, and forms a crest along the back and tail. For full description see A. Milne-Edwards, L Institut, xxxv. p. 46. Family 3. Rodents of various habit, but generally terrestrial ; with con tracted frontals, with the lower root of the maxillary zygomatic process more or less flattened into a perpendicular plate ; with a short and slender malar, generally reduced to a splint between the maxillary and squamosal processes ; with (in typical forms) a high, perpendicular infra-orbital foramen, wide above and narrow below ; with compressed lower incisors and rooted or rootless molars, tuberculate or with angular enamel folds. Premolars none (except in Sminthinas) ; pollex rudimentary ; tail generally sub-naked and scaly. This large family includes more than one-third of all the species of Rodents, and is represented by thirty-five genera of cosmopolitan distribution. These fall into two sections corresponding to the rooted or rootless condition of the molars. I. Molars rooted. Subfamily 1. Sminthinse. Pm , m f ; infra-orbital opening subtriangular, widest below; incisive foramina long. Sminthus, with one species, S. vagus, a Rat-like Rodent inhabit ing northern Asia and Europe. Subfamily 2. Hydromyinse. M f, divided into transverse lobes ; infra-orbital opening crescentic, scarcely narrowed above ;
incisive foramina very small.