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DORMOUSE

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Originally appearing in Volume V23, Page 446 of the 1911 Encyclopedia Britannica.
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DORMOUSE.) 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 JUMPING-MOUSE.) 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 JERBOA.) Mole-Rats.—The mole-rats (Spalacidae) bring us to the mouse-like section, or Myoidea, in which there are no premolars and the molars may be occasionally reduced to 1; these teeth being either rooted or rootless, with either cusps or enamel-folds, and the first gene-rally larger than the second. In the skull the zygomatic arch is slender and the fugal bone small and not extending far forwards, being supported by the long zygomatic process of the maxilla, while the infra-orbital foramen is mostly FIG. 9. Skull of the Muskrat (Fiber zibethicus). large, and there Natural size. 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. 442 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 MOLE-RAT.) 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. io). In the true zokors (Ellobius), on the other hand, From Milne-Edwards. 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 HAMSTER) ; 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 themolars 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 From Milne-Edwards. 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 VoLE (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 VoLE). Fiber—the muskrats—is a North American aquatic type (see MUSKRAT), characterized by the compression of the tail. Synaptomys is also North American, and characterized by the grooved After Gould. 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 LEMMING. Ellobius, which many naturalists place in this group, has been mentioned among the Spalacidae. 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 and have a plate-like structure, with the cusps or tubercles forming three longi- tudinal 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 A B hundred species, includes the true mice U Molars and rats (see MOUSE and RAT), and has of Mus (A) and Crice- incisors being narrow and smooth, the tus (B). 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 Philip-pines 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, de-creasing 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 and 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 origin-ally 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 Philip-pine 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. 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 than one pair of premolars, and the original ridges of all the cheek-teeth have become obscured and complicated by the development of secondary enamel-folds. The majority of these rodents, many of which are of large size, are terrestrial, but a few are burrowing, others arboreal and two or three aquatic. The Old World porcupines, constituting the family Hystricidae, are terrestrial, stoutly built rodents, with limbs of subequal length in front and behind, and the skin covered with strong spines. The upper lip is cleft, the jugal lacks an inferior angle, the fore part of the skull is short and broad; the cheek-teeth are partially rooted, with external and internal enamel-folds, the soles of the feet are smooth, there are six pairs of teats, the clavicles are imperfect and the tail is not prehensile. In the typical genus Hystrix, which prehensilis). is represented in all the three great continents of the Old World, and extends' as far east as Flores and Celebes, the skull is swollen and convex, the spines are cylindrical, and the tail is short and covered with spines and slender-stalked open quills. In Atherura fasciculata of the Malay Peninsula the spines are flattened, and the tails long and scaly, with a tuft of compressed bristles. A closely allied species, A. africana, inhabits Western Africa. The third genus is Trichys (see PORCUPINE). American Porcupines.—All the New World porcupines, representing the family Erethizontidae (or Coendidae) are arboreal in their habits, and have the upper lip undivided, the cheek-teeth rooted, the clavicles complete, the soles of the feet tuberculated and three pairs of teats. Erethizon dorsatus, the urson, is distributed all over the forest regions of North America; Synetheres (or Coendu) prehensilis, the prehensile-tailed porcupine of South America (fig. 15), represents a genus in which the whole upper surface of the body is protected by long white-tipped spines; Chaetomys subspinosus is clothed with strong wavy bristles. In the last two genera the feet have four toes, in place of the five of Erethizon (see PORCUPINE). Cavy Group.—In the family Caviidae, typified by the cavies (or guinea-pigs), may be included a large number of South and Central American rodents, among which the agoutis and pacas are often ranked as a family (Dasyproctidae) by themselves. The Caviidae, in the present more comprehensive sense, include the giants of the rodent order. Many of them, like ungulates, are specialized for swift running, and have unusually long limbs, with ridges developed on the articular surfaces of the lower bones; the clavicles are more or less reduced; the thorax is more compressed than usual, with a narrower breast-bone; and there is a marked tendency to the reduction or loss of the lateral toes, more especially in the hind limb. Since these rodents walk more or less entirely on their toes, in such a manner that the edges of the claws or nails come in contact with the ground, these tend to assume somewhat of a hoof-like character; while the foot-pads are more or less horny. The tail is generally very short, and its basal vertebrae are often fused with the sacrum. In the skull the lachrymal bone is large, the par-occipital process is directed vertically downwards and the tympanic bulla is hollow. In the soft parts the caecum is very large, the p,,nis is armed with a pair of barbed horny claspers and the scrotum is spiny. Special interest attaches to the most aberrant member of thefamily, the Peruvian Dinomys, known for more than thirty years only by a single specimen taken in a house in Lima, and only lately rediscovered. It is a large rodent known to the Tupi Indians as the paca-rana, or false paca, in allusion to the resemblance of its coloration to that of the true paca, from which it differs by its well-developed tail, the absence of cheek-pouches, the full development of all five toes and the wider thorax. The Tupi name may be adopted as the popular title of the species. Dr E. Goeldi states that the paca-rana is a rodent of phlegmatic and gentle disposition, which may account, perhaps, for its rarity, if, indeed, it be really scarce in its native home, which is probably the eastern slopes and tablelands of the Bolivian and Peruvian foot-hills bordering on Brazil, inclusive of the headwaters of the Purus, Acre and Jurua rivers. In the true pacas, Coelogenys (or Agouti), the first front toe is small, and both the first and fifth digits of the hind-foot are much inferior in size to the other three. The most remarkable feature of the genus is, however, the extraordinary development of the zygomatic arches of the skull, which are enormously expanded vertically, forming great convex bony capsules on the sides of the face, enclosing on each side a large cavity lined with mucous membrane internally, and communicating by a small opening with the mouth. C. paca is a white-spotted rodent, about 2 ft. long, and lives generally in the forests or along the banks of rivers (see PACA). The Agoutis, Dasyprocta, include several species of slender-limbed rodents, with three hind-toes, inhabiting Central and South America, one (D. cristata) extending into the West Indian islands. The members of both Coelogenys and Dasyprocta are terrestrial in their habits, and have the fore- and hind-limbs subequal, hoof-like claws, short or obsolete tail and rudimentary clavicles. • The masseteric ridge of the lower jaw is obsolete, the palate broad, the incisors long and the molars semi-rooted, with external and internal enamel-folds (see AGOUTI). The remaining and more typical members of the family, one of which is aquatic, are characterized by their short incisors, the strong masseteric ridges on the sides of the lower jaw, the long and curved par-occipitals and the palate contracted in front. Fore-feet with four digits, hind-feet with three; clavicles imperfect; molars divided by enamel-folds into transverse lobes; milk-teeth shed before birth. In the true cavies, or couies, Cavia, the fore- and hind-limbs are short and of subequal length, the ears are short and there is no tail. They include several species widely distributed throughout South America, extending even to the straits of Magellan, from one of which (C. cutleri of Peru) the guinea-pig is derived. The maras (Dolichotis) have the limbs and ears long and the tail very short. D. patagonica is a large species, nearly 3 ft. long, inhabiting the gravelly plains of Patagonia, while D. salinicola is a much smaller rodent from the salt-lagunas of Argentina. The palate is so much contracted in front that the premolars of opposite sides touch by their antero-internal edges. Hydrochaerus, in which all the feet are fully webbed, includes a single species, the capybara, or carpincho, the largest of living rodents. The skull (fig. 14) is distinguished not only by its great size, but by the enormous development of the par-occipital processes and the complex structure and large size of the last molars (see CAVY and CAPYBARA). Chinchilla Group.—The family, Chinchillidae, typified by the well-known chinchilla, includes a small number of South American rodents with large ears and proportionately great auditory bullae in the skull, elongated hind-limbs, bushy tails, very soft fur and perfect clavicles. The jugal is without an inferior angle, and extends forwards to the lachrymal; the palate is contracted in front and deeply emarginate behind; the incisors are short, and the molars divided by continuous folds into transverse plates; and the two halves of the lower jaw are welded together in front. It includes three existing genera, represented by some five species. Of these the true chinchilla, Chinchilla lanigera, C. brevicaudata, Lagidium perua, 'im and L. pallipes, are restricted to the alpine zones of the Andes _tom the northern boundary of Peru to the southern parts of Chili; while Lagostomus trichodactylus (or Viscaccia viscaccia), the viscacha, is confined to the pampas from the Uruguay river to the Rio Negro. In Chinchilla the fore-feet have five and the hind four digits, the tail is long and bushy, and the auditory bullae are enormous, appearing on the top of the skull ; Lagidiu-i has four digits in both fore- and hind-feet, and Lagostomus three only in the hind-feet, while the auditory bullae are much smaller (see CHINCHILLA and VISCACHA). Hutia Group.—The three remaining families of the Hystricoidea, of which one is African while the other two are chiefly South American, are very closely allied and often brigaded in a single family group. In the Capromyidae, which includes only the South American and West Indian hutias, the South American coypu and the African cane-rats, the tympanic bulla of the skull is hollow, the par-occipital process straight, the lachrymal small, and the cheek-teeth rooted, with deep enamel-folds; the first front toe being occasionally absent. Of the few living representatives of the group, the genus Myocastor (or Myopotamus) is represented only by the South American coypu, M. coypu, which is aquatic in its habits, and measures about 2 ft. in length, being the largest member of the group. It has a long tail, brown fur and red incisors, and lives in burrows near water, feeding on aquatic plants. The hutia (Capromys pilorides) is nearly as large, arboreal in habits, and a native of Cuba, where it is the largest indigenous mammal. Other species occur in Cuba, Jamaica and the Bahamas, while a Venezuelan species, Procapromys geayi, represents a separate genus. In one kind the tail is prehensile. All these rodents are remarkable for the manner in which the liver is divided into minute lobules. Plagiodontia aedium, another member of the group, is peculiar to Hayti. The African cane-rats, Thryonomys (or Aulacodus), are large terrestrial rodents, ranging from the centre of the continent to the Cape, easily recognized by their deeply fluted incisors (see CoYru). The Octodontidae, which are exclusively South American, differ from the preceding family by the tympanic bulla being filled with cellular bony tissue, and by the par-occipital process curving beneath it, while the cheek-teeth are almost or completely rootless and composed of parallel plates. The first front toe may be absent. The more typical members of the family are rat-like burrowing rodents, living in communities. The typical genus is represented by the degu (Octodon degus) and several nearly related species; other genera being Ctenomys, Octodontomys (Neoctodon), Aconaemys, Spalacopus and Abrocoma; the latter taking its name from its unusually soft fur. Among these, the tuco-tucos (Ctenomys) are characterized by their burrowing habits, almost rudimentary ears, small eyes, short tails and the kidney-shaped grinding-surfaces of their cheek-teeth. They take their name of tuco-tuco from their cry, which resembles the blows of a hammer on an anvil, and may be heard all day as the little rodents move in their burrows, generally formed in sandy soil. In some districts the ground is undermined by these burrows, in which stores of food are accumulated. The species of Octodon have larger ears, longer, tufted tails and the sides of the cheek-teeth indented by plates of enamel; they are chiefly found in hedgerows and bushes, where they burrow. In Abrocoma the tail has no tuft, the ears are still larger and the lower cheek-teeth more complex than the upper ones. Aconaemys is an allied Chilean genus in which the enamel-folds meet across the molars. Several of these rodents live in the Andes, where the ground is covered for months with snow. The second group of the family is formed by the genera Loncheres, Dactylomys, Echi[nolmys, Proechimys and a few others, the members of which are rat-like rodents, with long scaly or furry tails, and frequently flattened spines mingled with the fur of the back. Most species are brown above and whitish beneath, but in some the lighter tints extend on to the sides, shoulders and head, communicating a coloration somewhat like that of a guinea-pig (see OcTOnoN). The North African gundis (Ctenodactylus gundi and Ct. vali) are the types of an African family, which also includes the genera Massoutiera, Pectinator and Petromys. In the gundi the two inner toes of the hind-foot are furnished with a horny comb and bristles for the purpose of cleaning the fur, and the tail is very short; but in Pectinator the tail is longer. Petromys has a still longer and more bushy tail, and no comb to the hind-feet. The gundi is a diurnal species, inhabiting rocky districts, and having habits very similar to those of a jerboa. Of these Ctenodactylus and Pectinator are characterized by the union of the incus and malleus of the internal ear, the free fibula and the almost rootless cheek-teeth. The premolar is very small, thus showing an approximation to the Myoidea, although in other respects Petromys appears to approximate to the Hystricidae. Picas and Hares.—The remaining rodents, which include two families—the picas (Ochotonidae) and the hares and rabbits (Leporidae)—constitute a second sub-order, the Duplicidentata, differing from all the foregoing groups in possessing two pairs of incisors in the upper jaw (of which the second is small, and placed directly behind the large first pair), the enamel of which extends round to their posterior surfaces. At birth there are three pairs of incisors, but the outer one is soon lost. The incisive foramina are large and usually confluent; the bony palate is very narrow from before backwards; there is no alisphenoid canal; the fibula is welded to the tibia, and articulates with the calcaneum; and the testes are permanently external. All are terrestrial, and in many cases burrowing, in their habits, and some of them are of extreme fleetness. The Ochotonidae are represented at the present day only by the single genus Ochotona (Lagomys), which includes all the picas, or mouse-hares. They are small rodents with complete clavicles, fore- and hind-limbs of nearly equal length, no external tails and short ears. Skull depressed, frontals contracted and without post-orbital processes ; p. or 4; molars rootless, with transverse enamel-folds. In some cases the molar-formula is ;. The genus includes about a score of species of guinea-pig-like animals, inhabiting chiefly the mountainous parts of Northern Asia (from i1,000 to 14,000 ft.), one species only being known from South-east Europe and several from the Rocky Mountains and Alaska. From the picas the hares and rabbits (Leporidae) are distinguished by the imperfect clavicles, the more or less elongated hind-limbs, short recurved tail (absent in one case) and generally long ears. The skull is compressed, with large wing-shaped post-orbital processes (fig. 16) ; p. With the exception of Australasia, the family has a cosmopolitan distribution; and its numerous species resemble one another more or less closely in general externalcharacters. In all the fore-limbs have five and the hind four digits; and the soles of the feet are densely clothed with hairs similar to those covering the legs; the inner surface of the cheeks being hairy. Although the family has such a wide dis- tribution, the greater number of the species are restricted to Europe, north-ern and central Asia and North America; South America having very few. Till within the last few years the majority of naturalists followed the practice of including all the members of the family in the genus Lepus. It is true that Mr E. Blyth long ago proposed the name Caprolagus for the remarkable spiny rabbit of the western Himalayas, while the generic name Oryctolagus was suggested later for the rabbit, and Sylvilagus for the American " cotton-tails "; but none of these was accorded general acceptation. Of late years, however, zoologists have come to the conclusion that generic sub-divisions of the Leporidae are advisable. In 1899 Dr Forsyth Major proposed a classification of the family in which a number of species were grouped with the spiny rabbit in the genus Caprolagus, whilst Oryctolagus was taken to include not only the common rabbit, but likewise the Cape hare. A more recent classification is that of Mr M. W. Lyon, in which by far the largest number of species of the family are retained in the original genus Lepus, which has also the widest geographical distribution of all the genera. It is typified by the blue hare (Lepus timidus), next to which comes the common hare (L. europaeus) and certain other allied forms. The jackass-hares of Mexico, &c., such as L. californicus, form a second sub-group; while these are in turn followed by the American hare (L. americanus) and its immediate relatives. The cotton-tails, or wood-rabbits, of North and South America are regarded as forming a genus, Sylvilagus, by themselves, which includes the Brazilian and Paraguay hares, and appears to be chiefly distinguished by a certain feature in the parietal region of the skull. Under the name of Oryctolagus cuniculus, the rabbit is considered to represent a genus by itself, specially characterized by the shortness of the ears and hind-feet. The swamp-rabbit (L. palustris) and water-hare (L. aquaticus) of the southern United States form the group Limnotragus, characterized by the harsher fur, the shorter ears, tail and hind-feet, and the complete fusion of the post-orbital process (which is so distinct in the typical hares) with the adjacent parts of the skull, so that neither notches nor perforations are developed in this region. The short-tailed rabbit of the western United States (Brachylagus idahoensis) is the sole member of a group allied in general characters to the typical Lepus, but distinguished by the unusually short tail. Another group is Pronolagus, typified by the Cape thick-tailed hare, the so-called Lepus crassicaudatus, which is externally similar to Lepus proper, but has the skull and teeth of the general type of the next group. The tail-less rabbit of Mount Popocatepetl, Mexico, originally described as a distinct generic type, under the name of Romerolagus nelsoni, is broadly distinguished by the entire absence of the tail, and the short ears and hind-feet, its general form being like that of the Liu-Kiu rabbit, while, as in the latter, the post-orbital process of the skull is small, and represented only by the hinder half. Next come three remarkable rabbits from the Indo-Malay countries, all closely allied, although regarded as representing three generic groups, Nesolagus, Caprolagus and Pentalagus. In all three the skull is of the type of Romerolagus. The first is represented by the Sumatran rabbit, the so-called N. netscheri, which apparently differs from the spiny rabbit mainly by the pattern of the cheek-teeth. The spiny rabbit, separated from Lepus by Blyth in 1845 under the name of Caprolagus hispidus, is an inhabitant of Assam and the adjacent districts, and distinguished by its harsh, bristly fur and short ears and tail. In the Liu-Kiu rabbit (Pentalagus furnessi) the coat is equally harsh, but the ears and hind-feet are shorter, and there are only five (in place of the usual six) pairs of upper cheek-teeth. In the loss of the last upper molar, the Liu-Kiu rabbit approximates to the picas, as does the tailless rabbit in the abortion of its caudal appendage. Mr Lyon's scheme seems to be the best attempt to explain the affinities of the members of the group. Whether all his genera be adopted, or all the species be included in Lepus, must largely be a matter of individual opinion. If the latter course be followed, Mr Lyon's genera must be reduced to the rank of sub-genera, and his sub-generic divisions of Lepus and Sylvzlagus ignored. (See HARE and RABBIT.)
End of Article: DORMOUSE
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