Chapter X: The Order Rodentia (1)

The Rodentia, or Rodents, form a well-defined order, readily distinguished by their large scalpriform incisors and the absence of any trace of canines. The existing forms are mostly of comparatively small size, and are generally of terrestrial habits, although a few are arboreal or natatorial. The dentition is diphyodont; the mandible never has more than a single pair of incisors; the premolars are always below the full number, being very generally ¹⁄₁, or altogether wanting. The feet are plantigrade or semi-plantigrade, generally with five digits, and usually unguiculate, although occasionally of a subungulate type. Clavicles are present as a rule, although they may be imperfect or rudimentary.

The upper incisors resemble the lower in growing uninterruptedly from persistent pulps, and, except in the suborder Duplicidentata, agree with them in number; the premolars and molars may be rooted or rootless, with tuberculated or laminated crowns, and are arranged in an unbroken series. The orbits communicate freely with the temporal fossæ; the condyle of the mandible is elongated in the antero-posterior direction, and, through the absence of a post-glenoid process to the squamosal, admits of a backward and forward motion of the jaw. The intestine (except in the _Myoxidæ_) has a large cæcum; the testes are inguinal or abdominal; the uterus is two-horned, the cornua either opening separately into the vagina or uniting to form a corpus uteri; the placenta is discoidal and deciduate; and the smooth cerebral hemispheres do not extend backwards so as to cover any part of the cerebellum.

The Rodents include by far the greatest number of species, and have the widest distribution of any of the orders of terrestrial mammals, being in fact cosmopolitan, although more abundant in some parts than in others. The total number of known existing species exceeds 900. South America may be regarded as their headquarters at the present day; while in Australia and Madagascar they are represented only by a few genera. All the Rodents are exclusively herbivorous, and the whole of them gather their food by gnawing. They present considerable diversity of habits. Thus the Squirrels are arboreal, and some of them provided with a parachute for taking flying leaps from tree to tree; the Hares are cursorial; the Jerboas agile jumpers; the Mole-Rats fossorial; while the Beavers and Water-Voles are aquatic. In spite, however, of this diversity of habits the Rodents present a remarkable similarity in general structure; so much so, indeed, that the characters employed for distinguishing the various families and genera are comparatively trivial, and of slight structural importance. The skull of the Rodents is characterised by the invariable presence of the zygomatic arch, of which the middle portion is formed by the jugal (Fig. 7, p. 37); and, as already mentioned, the orbit communicates freely with the temporal fossa. There is invariably a long diastema separating the incisors from the cheek-teeth; and, with the exception of the Duplicidentata, the glenoid cavity of the squamosal is elongated antero-posteriorly. Postorbital processes of the frontals exist only in the Squirrels, Marmots, and Hares; in all other genera they are rudimentary or altogether absent; the zygoma never sends upwards a corresponding process; the lachrymal foramen is always within the orbital margin; in many species the infraorbital foramen is very large (in some as large as the orbit), and transmits part of the great masseter muscle (Fig. 194, _m_), by means of which the jaws are worked. The zygomatic arch varies in its degree of development, and the position of the jugal therein is used as a distinguishing character for grouping the families; the nasals are, with few exceptions, large, and extend far forwards; the parietals are moderate, and there is generally a distinct interparietal. The palate is narrow from before backwards—this being especially pronounced in the Hares, where it is reduced to a mere bridge between the premolars; while in other cases, as in the Mole-Rats (_Bathyerginæ_), it is extremely narrow transversely, its width being less than that of one of the molar teeth. Auditory bullæ are always present, and generally large; in some genera, as in the Gerbilles and Jerboas, there are also supplemental mastoid bullæ forming great hemispherical bony swellings at the back of the skull (see Fig. 7, _Per_); and in these genera, and in the true Hares, the meatus auditorius is tubular and directed upwards and backwards. The mandible is characterised by the abruptly narrowed and rounded symphysial part supporting the large incisors, as well as by the small size of the coronoid process and the great development of the angular portion.

The dental formula varies from _i_ ²⁄₁, _c_ ⁰⁄₀, _p_ ³⁄₂, _m_ ³⁄₃ (total 28) in the Duplicidentata to _i_ ¹⁄₁, _c_ ⁰⁄₀, _p_ ⁰⁄₀, _m_ ²⁄₂ (total 12) in _Hydromys_, _Xeromys_, and one species of _Heterocephalus_; but in the great majority of forms it is very constant, _i_ ¹⁄₁, _c_ ⁰⁄₀, _p_ ⁰⁻¹⁄₀₋₁, _m_ ³⁄₃ being very typical. Only in the Duplicidentata is there a second pair of upper incisors, which are of very small size, and situated immediately behind the large normal pair. This group is also peculiar in that the enamel of the incisors is not confined to their anterior surfaces, but extends partially on to their sides. It is by reason of the thick layer of enamel on their anterior surface and its absence from the posterior surface that the incisors maintain their sharp chisel-like edge, which is so essentially characteristic of the order. Both the upper and the lower incisors are regularly curved—the curvature being somewhat greater in the upper ones—and since they grew continuously from persistent pulps, it is quite evident that should any accident, such as the loss of one of them, or displacement by fracture of the jaw, prevent the regulation of the length by attrition against one another, the unopposed tooth will gradually curve upon itself until a complete circle or more has been formed, the tooth, perhaps, passing during its growth through some part of the animal’s head. The molars, as already mentioned, may be rooted or rootless, tuberculated or laminated; this diversity of structure occurring even in the same family. When there are more than three cheek-teeth those in front of the last three have succeeded milk-teeth, and must therefore be considered premolars. In some species, as in the Agoutis (_Dasyproctidæ_), the milk-teeth are long retained, while in the allied Cavies (_Caviidæ_) they are shed before birth.

There are generally nineteen dorso-lumbar vertebræ (thirteen dorsal and six lumbar), their form varying in the different genera. In the cursorial and leaping species the lumbar transverse processes are generally very long, and in the Hares there are large compressed hypapophyses. The caudal vertebræ exhibit great variety in structure, being in a rudimentary condition in the Guinea-Pig, while in the Jumping Hares and prehensile-tailed Porcupines they are of very large dimensions. The scapula is usually narrow, with a long acromion; the clavicles may be altogether absent or imperfect, as in the Porcupines, Cavies, and Hares, but in most species they are well developed. In all existing forms the humerus has no entepicondylar foramen, and the radius and ulna are distinct. In most species the manus has five digits, with phalanges normally developed; the pollex being rarely rudimentary or absent. The pelvis has well-developed ischia and pubes, meeting in a long, and usually bony, symphysis. The femur varies considerably in form, but generally has a well-defined third trochanter; in the Sciurine and Hystricine Rodents the tibia and fibula are distinct, but in the Rats and other Murines, and in the Hares, these bones are united, often high up; the pes is much more variable than the manus, the digits varying in number from five, as in the Squirrels and Rats, to four, as in the Hares, or even three, as in the Capybara, Viscacha, and Agouti; in the _Dipodidæ_ the metatarsals are greatly elongated, and in some of the species, as in the Jerboas, they are ankylosed together.

The mouth is divided into two cavities communicating by a constricted orifice, an anterior one containing the large incisors, and a posterior one in which the molars are placed; the hairy integument of the face being continued inwards behind the incisors. This peculiar arrangement evidently prevents substances not intended for food getting into the mouth, as when the animal is engaged in gnawing through an obstacle. In the Hares and Pacas the inside of the cheeks is hairy, and in some species, as in the Pouched Rats and Hamsters, there are large internal cheek-pouches lined with the hairy integument, which open near the angles of the mouth and extend backwards behind the ears. In the New World Pouched Rats (_Geomyidæ_) the pouches open externally on the cheeks. The tongue presents little variability in length, being always short and compressed, with an obtuse apex never protruded beyond the incisors. In most species there are three circumvallate papillæ at the base; and the apical portion is generally covered with small filiform papillæ, some of which in the Porcupines (_Hystrix_) become greatly enlarged, forming toothed spines. The stomach varies in form from the simple oval sac of the Squirrel to the complex ruminant-like organ of the Lemming. In the Water-Vole (_Arvicola amphibius_) and the Agouti (_Dasyprocta aguti_) it is strongly constricted between the œsophagus and pylorus. In the common Dormouse the œsophagus immediately before entering the stomach is much dilated, forming a large egg-shaped sac with thickened glandular walls; and in some other species, as in _Lophiomys imhausi_ and in the Beaver, glandular masses are attached to and open into the cardiac or pyloric pouches. The alimentary canal (Fig. 196) of all Rodents, with the exception of the Dormice (_Myoxidæ_), has a cæcum, which is often of great length and sacculated, as in the Hares, Water-Voles, and Porcupines. In some instances, as in the Hamster and Water-Vole, the long colon is spirally twisted upon itself near its commencement. The liver is typically divided in all, but the lobes are variously subdivided in the different species (in _Capromys_ they are divided into minute lobules); and the gall-bladder, though present in most, is absent in a few. In most species the penis (which is generally provided with a bone) can be more or less completely retracted within the fold of integument surrounding the anus, where it lies curved backwards upon itself under cover of the integument. It may, however, be carried forward some distance in front of the anal orifice, from which in the breeding season, as in the Voles and Marmots, the prominent testicular mass separates it. The testes in the rutting season form projections in the groins, but (except in the Duplicidentata) do not completely leave the cavity of the abdomen. Prostatic glands and, except in the Duplicidentata, vesiculæ seminales are present in all. The uterus may be double, each division opening by a separate aperture into a common vagina, as in _Leporidæ_, _Sciuridæ_, and _Hydrochœrus_, or completely two-horned, as in most species. The mammæ vary in number and position from the single abdominal pair of the Guinea-Pig to the ten thoracico-abdominal pairs found in some of the Rats. In the _Octodontidæ_ the mammæ are placed high up on the sides of the body.

The peculiar odour evolved by many Rodents is due to the secretions of special glands, which may open either into the prepuce, as in _Mus_, _Arvicola_, _Cricetus_, etc., or into the rectum, as in _Arctomys_ and _Aulacodus_ or into the passage common to both, as in the Beaver, or again, into pouches opening near the anus, as in the Hare, Agouti, and Jerboa.

The integument is generally thin, and the panniculus carnosus (the sheet of muscle underlying the skin) rarely much developed. The fur varies exceedingly in character. Thus it may be very fine and soft, as in the Chinchillas and Hares, in others more or less replaced by spines on the upper surface, as in the Spiny-Rats and Porcupines; in several genera, as in _Xerus_, _Acanthomys_, _Platacanthomys_, _Echinothrix_, _Loncheres_, and _Echinomys_, the spines are flattened. In the muscular structures the chief peculiarities are noticeable in the comparatively small size of the temporal muscles, and in the great double masseters (Fig. 194), which are the principal agents in gnawing; the digastrics also are remarkable for their well-defined central tendon, and in many species their anterior bellies are united between the mandibular rami; the cleidomastoid generally arises from the basioccipital, and the pectoralis major is connected with the latissimus dorsi; in the Porcupines and Hares the tendons of the flexor digitorum longus and flexor hallucis longus are connected in the foot, while in the Rats and Squirrels they are separate, and the flexor digitorum longus is generally inserted into the metatarsal of the hallux.[290]

Rodents are tolerably well represented in a fossil condition from the period of the Upper Eocene, while if _Decticadapis_, of the Lower Eocene of Rheims, is rightly referred to it the order dates from the oldest Tertiary. All the fossil forms at present known are, however, essentially true Rodents, and afford no clue as to the relations of the order with other mammals. The remote affinities of the Rodents to the Proboscidea, as well as their more marked resemblances to _Typotherium_, have been already mentioned. Whether there is a real genetic affinity (as Professor Cope suggests) with the Tillodontia cannot be decided with the evidence at present available.

_Suborder_ SIMPLICIDENTATA.

Only one pair of upper incisors, having their enamel confined to their front surfaces. Incisive foramina moderate and distinct; fibula not articulating with the calcaneum. Testes abdominal, and descending periodically only into a temporary sessile scrotum.

_Section_ SCIUROMORPHA.

Zygomatic arch slender, chiefly formed by the jugal, which is not supported by a long maxillary process extending backwards beneath it; postorbital processes of frontal present or absent; infraorbital opening small (except in _Anomalurus_); mandible with the angular part arising from the inferior surface of the bony socket of the lower incisor; clavicles well developed; fibula distinct.

_Family_ ANOMALURIDÆ.

Arboreal forms, having their limbs connected by a cutaneous expansion supported by a cartilaginous process arising from the olecranon; tail long and hairy, with large imbricated scales on its inferior surface near the root; sixteen pairs of ribs; no postorbital processes on the frontals; _p_ ¹⁄₁; molars not tuberculate, with transverse enamel-folds. Confined to the Ethiopian region.

_Anomalurus_,[291] with several species from West and Central Africa, alone represents the family. The peculiar caudal scales, which evidently assist the animal in climbing, and the position of the cartilaginous support of the parachute, are well shown in Fig. 197. All the species but two are from Western Africa; _A. orientalis_ occurs near Zanzibar, and _A. pusillus_ is from the equatorial regions of that continent. According to Mr. O. Thomas,[292] the latter “little animal is most nearly allied to the West-African _A. beecrofti_, but differs from that species in its duller and less yellow upper side, in the entire absence of rufous on its neck and belly, and, as from all the other described species, in its diminutive size.”

_Family_ SCIURIDÆ.

Arboreal or terrestrial forms, with cylindrical hairy tails, without scales, and with twelve or thirteen pairs of ribs. Skull (Figs. 198, 199) with distinct postorbital processes; infraorbital opening small; palate broad; _p_ ²⁄₁; first upper premolar very small or deciduous; molars rooted, tubercular.

Subfamily =Sciurinæ=.—Incisors compressed; form slender; tail long and hairy. Cosmopolitan (excluding Australian region).

This subfamily includes the true Squirrels, of which seven existing genera are usually recognised.

_Sciurus._[293]—Tail long and bushy; ears generally well developed, pointed, often tufted; feet adapted for climbing, the anterior having four digits and a rudimentary pollex, and the posterior with five digits, all of which have long, curved, and sharp claws. Mammæ, from four to six. Skull (Fig. 199) lightly built, with long postorbital processes. Penultimate upper premolar, when present, minute.

True Squirrels are found in most of the temperate and tropical regions of the world, exclusive of Madagascar and the Australian region. They are, however, most abundant in the Malayan part of the Oriental region, and attain their largest size and most brilliant coloration in the tropics. Their size is very variable, so that whereas _S. soricinus_, of Borneo, is no larger than a Mouse, _S. bicolor_, of the Malayan region, is nearly as large as a Cat. The common English Squirrel (_S. vulgaris_) is found over the whole of the Palæarctic region, reaching in one direction from Ireland to Japan, and in the other from the north of Italy to Lapland; its remains occur in the Norfolk “Forest-bed.” In the Malayan region “nearly all the numerous species are brilliantly marked, and many are ornamented with variously coloured longitudinal stripes along their bodies. One of the commonest and best known of the striped species is the little Indian Palm-Squirrel (_S. palmarum_), which in large numbers runs about every Indian village. Another Oriental species (_S. caniceps_) presents almost the only known instance among mammals of the temporary assumption during the breeding season of a distinctly ornamental coat, corresponding to the breeding-plumage of birds. For the greater part of the year the animal is of a uniform gray colour; but about December its back becomes a brilliant orange-yellow, which lasts until about March, when it is again replaced by gray. The Squirrel shown in Fig. 200 is a native of Burma and Tenasserim, and is closely allied to _S. caniceps_, but goes through no seasonal change of colour.

“The number of species in the genus is about 75, of which 3 belong to the Palæarctic, 15 to the Ethiopian, about 40 to the Oriental, and 16 to the combined Nearctic and Neotropical regions” (Thomas).

Fossil species referred to _Sciurus_ are found in the European Tertiaries down to the Phosphorites of Central France, while others occur in the White River Miocene of the United States.

_Rhithrosciurus._[294]—A very striking Squirrel, confined to Borneo, and as yet only known from three or four examples, has been separated generically under this name. The general shape of its skull is very different from that of other Squirrels; but its most peculiar characteristic is the presence of from seven to ten minute parallel vertical grooves running down the front face of its incisors; no other Squirrel having really grooved incisors, and no other member of the whole order incisive grooves resembling these. Its premolars number ¹⁄₁, and its molars are simpler and less ridged than in the other genera. This Squirrel (_R. macrotis_) is far larger than the English, with an enormously long bushy tail, long tufted ears, and black and white bands down its sides.

_Xerus._[295]—Fur coarse and spiny. Claws long and comparatively straight. Ear-conchs minute or absent. Skull with the postorbital processes short and directed backwards, the bony palate prolonged considerably behind the tooth-row, and the external ridge on the front face of the anterior zygomatic root more developed, and continued much farther upwards than in _Sciurus_. Premolars ²⁄₁; molars as in _Sciurus_. Mammæ two. This genus contains four well-marked species, known as Spiny Squirrels, all natives of Africa. They are terrestrial in their habits, living in burrows which they dig for themselves. _X. getulus_, a striped species of North Africa, has much the size and appearance of the Indian Palm-Squirrel; all the others are a little larger than the English Squirrel.

_Tamias._[296]—All the members of this genus are characterised by the possession of internal cheek-pouches, and by their style of coloration; being ornamented on the back with alternate light and dark bands. Their skulls are slenderer and lighter than those of the true Squirrels, from which they differ in several unimportant details. There is only one functional premolar—the small anterior one usually found in _Sciurus_ being either absent altogether or quite small and functionless. There are some four well-defined species, all found in North America, one (_T. asiaticus_) extending also through Siberia into Eastern Europe.[297] They are generally known as Ground-Squirrels, but in America, where they are among the commonest and best known of the indigenous Rodents, as “Chipmunks.” The members of this genus seem to lead into the genus _Spermophilus_, so that the division of the _Sciuridæ_ into two subfamilies, although convenient for classification, is rather artificial.

Remains of _Tamias_, probably belonging to existing species, occur in the Pleistocene deposits of Europe and Nebraska.

_Pteromys_[298] and _Sciuropterus_.[299]—The Flying Squirrels, although incapable of true flight, can yet float through the air for considerable distances by the aid of an extension of skin connecting their fore and hind limbs, and forming a sort of parachute. This parachute is merely a lateral extension of the ordinary skin of the body, which passes outwards between the limbs and terminates at the wrists and ankles. In addition to the lateral membrane there is a narrow and inconspicuous one passing from the cheek along the front of the shoulder to the front of the wrist, and another—at least in the larger species—stretching across behind the body from ankle to ankle and involving the base of the tail. The Flying Squirrels are divided into three genera. Of those with a normal dentition _Pteromys_ contains the larger and _Sciuropterus_ the smaller species. The two differ in certain details of dentition, as well as in the greater development in the former of the expanded membranes, especially of the “interfemoral” or posterior membrane, which in the latter is almost wholly absent. In _Pteromys_ the tail is cylindrical and comparatively thin, while in _Sciuropterus_ it is broad, flat, and laterally expanded, and evidently compensates for the absence of the interfemoral membrane by acting as a supplementary parachute. In appearance Flying Squirrels resemble the other forms, although they are even more beautifully coloured. Their habits, food, etc., are also very similar to those of the true Squirrels, except that they are more decidedly nocturnal, and are therefore less often seen by the traveller; their peculiar shrill cry is, however, well known to all who have camped out in the regions which they inhabit. Their mode of flight is precisely similar to that of the Flying Phalangers of Australia. Of each of the two genera there are about thirteen or fourteen species, all natives of the Oriental region, except that one of _Sciuropterus_ is found in North America, and another in Siberia and Eastern Europe.

_Eupetaurus._[300]—Externally as in _Pteromys_, except that the claws are less sharp. Skull with a more produced muzzle than in the latter, more distinct supraorbital notches, longer anterior palatal foramina, and a shorter bony palate. Cheek-teeth differing from those of all other _Sciuridæ_ in their hypsodont character. One large species (_E. cinereus_), from Gilgit and adjacent districts on the extreme north-west of Kashmir territory. This fine Flying Squirrel is chiefly known by one entire specimen and some imperfect skins.

_Extinct Genera._—The genera _Pseudosciurus_ and _Sciuroides_, from the Upper Eocene of Europe, have the molar teeth more elongated than in _Sciurus_. _Gymnoptychus_ with _p_ ¹⁄₁, from the North American Miocene, approximates in the structure of its molars to _Tamias_. _Meniscomys_ (_p_ ²⁄₁), from the latter deposits, together with _Sciurodon_ of the French Phosphorites, are regarded as Squirrels showing signs of affinity with the _Haplodontidæ_.

Subfamily =Arctomyinæ=.—Incisors not compressed; typically the form stout, and the tail comparatively short. This subfamily comprises burrowing forms which may be collectively known as Marmots; as already mentioned, they are so intimately connected with the preceding subfamily that the division into two groups is purely a matter of convenience. They are confined to the Palæarctic and Nearctic regions.

_Arctomys._[301]—External form stout and heavy, ears short, tail short and hairy, cheek-pouches rudimentary or absent. Fore feet with four well-developed digits, and a rudimentary pollex provided with a flat nail. Skull (Fig. 198) large and heavy, with the postorbital process stout, and at right angles to the axis. Incisors broad and powerful. First upper premolar nearly as large as the second. Molar series nearly parallel, scarcely converging behind at all.

The various species of true Marmot, which exceed a dozen in number, are all much alike in general appearance, ranging in size from about 15 to 25 inches in length, with tails from 3 to 12 inches long.

The Alpine Marmot (Fig. 201) is peculiar to Europe, being found in the Alps, Pyrenees, and Carpathians; its remains occur in European Pleistocene deposits. _A. bobac_ occurs in Eastern Europe and Siberia. Several species (_e.g._ _A. monax_, Fig. 198) are found in the Nearctic region, and many in Kashmir and Central Asia. The long-tailed Red Marmot (_A. caudatus_) is a fine Himalayan species, which may be seen on the mountain passes to the north of the valley of Kashmir, as soon as the snow begins to disappear, sitting at the entrance to its burrow, which is generally beneath a rhubarb plant.

The following account of the habits of the Alpine Marmot is given by Professor Blasius: “Marmots live high up in the snowy regions of the mountains, generally preferring exposed cliffs, whence they may have a clear view of any approaching danger, for which, while quietly basking in the sun or actively running about in search of food, a constant watch is kept. When one of them raises the cry of warning, the loud piercing whistle so well known to travellers in the Alps, they all instantly take to flight and hide themselves in holes and crannies among the rocks, often not reappearing at the entrance of their hiding-places until several hours have elapsed, and then frequently standing motionless on the look-out for a still longer period. Their food consists of the roots and leaves of various Alpine plants, which, like squirrels, they lift to their mouths with their fore paws. For their winter quarters they make a large round burrow, with but one entrance, and ending in a sleeping-place thickly lined with hay. Here often from ten to fifteen Marmots pass the winter, all lying closely packed together fast asleep until the spring.”

_Cynomys._[302]—Size and form intermediate between _Arctomys_ and _Spermophilus_. Ears and tail short. Cheek-pouches shallow. Fore feet with five claws, that on the pollex as large as that on the fifth toe. Skull (Fig. 202) heavily built, with the postorbital processes directed outwards. Dentition (as shown in Fig. 202) remarkably heavy, the molar teeth differing from those of _Arctomys_ and _Spermophilus_ by having three instead of two transverse grooves on their crowns. First premolar nearly as large as the second. Molar series strongly convergent behind.

Two species of Prairie Marmots, or, as they are often called, “Prairie-Dogs,” are found in North America. They live together in large communities, inhabiting burrows excavated at short distances apart, and feeding 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 habitation, the latter coming there to feed on the young Marmots.

_Spermophilus._[303]—Size much smaller than in either of the preceding genera; form more slender and squirrel-like. Tail very variable, from 1 to 8 or 9 inches in length. Cheek-pouches always present. Fore feet with four well-developed toes and a rudimentary pollex, of which the claw may be either present or absent. Skull more lightly built than in the other preceding genera, with the postorbital processes slender and directed backwards. Molar series nearly parallel, as in _Arctomys_, but all these teeth much smaller and lighter; first premolar simply rounded, never more than about one-third of the size of the second.

The Pouched Marmots, or Sousliks, have nearly the same distribution as _Tamias_, and are represented by a considerable number of species. They present a far greater range of variation than is found among the true Marmots, some of them, such as the European species, being scarcely as large as a common squirrel, almost entirely without external ears, and with the tail reduced to a mere stump, barely an inch long, while others are more than three times this size, with large and often tufted ears, and long bushy squirrel-like tails. Professor Blasius gives the following details of the habits of the common European Souslik (_S. citillus_): “It lives in dry treeless plains, especially on a sandy or clayey soil, and is never found either in forests or on swampy ground. It forms burrows, often 6 or 8 feet deep, in which food is stored up and the winter sleep takes place. Each burrow has but one entrance, which is closed up when winter approaches,—a second hole, however, being previously formed from the sleeping-place to just below the surface of the ground. The second hole is opened the next year, and used as the ordinary entrance, so that the number of closed-up holes round a burrow gives an indication of the length of time that it has been occupied. Sousliks ordinarily feed on roots, seeds, berries, etc., but occasionally also on animal food, preying readily on eggs, small birds, and mice, the remains of these latter being often found in their burrows. They bring forth in the spring from four to eight young ones, which, if taken early, may be easily tamed. They are often eaten by the peasants, the inhabitants of the Russian steppes considering their flesh an especial delicacy.”

Remains of _Spermophilus_ are not uncommon in European Tertiary deposits, some belonging to living and others to extinct species.

_Extinct Genera._—_Plesispermophilus_, from the Upper Eocene Phosphorites of Central France, appears to be closely allied to the Sousliks. _Plesiarctomys_ (_Sciuravus_ or _Paramys_), which is common to the Middle Tertiaries of Europe and North America, appears to be a generalised form, showing some resemblance both to _Arctomys_ and _Sciurus_, but with tritubercular upper molars and no postorbital processes to the skull; in the latter respect agreeing with the next family. In the size of the preorbital vacuity the skull resembles the Hystricomorpha.

_Family_ HAPLODONTIDÆ.

Distinguished from the _Sciuridæ_ by the absence of postorbital processes to the frontals, the depressed skull, and the rootless cheek-teeth. Premolars ²⁄₁; the penultimate upper one small.

_Haplodon._[304]—_H. rufus_ and _H. major_, of North America, west of the Rocky Mountains, are the only representatives of the family; their habits are similar to those of _Cynomys_.

_Family_ CASTORIDÆ.

Skull massive, without postorbital processes, the angle of the mandible rounded, and the cheek-teeth rootless, with re-entering enamel-folds. Premolars ¹⁄₁. Habits natatorial.

_Castor._[305]—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 œsophageal 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. One or two species, Palæarctic and Nearctic.

Zoologists are not yet of accord as to whether the European and American Beavers should be regarded as distinct species or as local races; the general concensus of opinion being in favour of the latter view.

The European Beaver (_C. fiber_) was at one time an inhabitant of the British Isles, having been found, according to Pennant, in certain Welsh rivers so late as the twelfth century, while subfossil remains of it occur in the peat-beds of many parts of the country. In Scandinavia Beavers are still found in the neighbourhood of Arendal. Isolated pairs are occasionally met with on the banks of the Rhone, Weser, and Elbe; and a considerable number are kept in a park belonging to the Emperor of Austria, on the banks of the Danube. They also occur sparingly in Russia and Poland, in the streams of the Ural Mountains, and in those which flow into the Caspian. They live in burrows on the banks of rivers, like the Water-Rat, and show little of the architectural instinct so conspicuous in the American form, but this may be owing to unfavourable external conditions rather than to want of the faculty; for there is a well-authenticated instance of a colony of Beavers, on a small stream near Magdeburg, whose habitations and dam were exactly similar to those found in America.

The American Beaver (_C. canadensis_) extends over that part of the American continent included between the Arctic circle and the tropic of Cancer; owing, however, to the gradual spread of population over part of this area, and still more to the enormous quantity of skins that, towards the end of last and the beginning of the present century, were exported to Europe, numbering about 200,000 annually, this species is in imminent danger of extirpation. It is distinguished from the European Beaver by the shorter and somewhat wider nasals.

Remains of extinct species of _Castor_ occur in the Pliocene of Europe, and in the North American Miocene; the one from the last-mentioned deposits being of small size, and separated by some writers as _Eucastor_.

_Extinct Genera._—A very large Beaver known as _Trogontherium_ (_Diobroticus_), and distinguished by the nature of the enamel-folds of the molars, occurs in the Upper Pliocene and Pleistocene of Europe. _Chalicomys_ (_Steneofiber_) is a considerably smaller form from the Miocene of Europe and the United States, distinguished from all existing Rodents by the presence of an entepicondylar foramen in the humerus. _Palæocastor_, of the North American Miocene, is allied.

_Section_ MYOMORPHA.

Skull (Fig. 203), with slender zygomatic arch, in which the jugal seldom extends far forwards, being usually supported by the long zygomatic process of the maxilla; no postorbital process; infraorbital vacuity variable; angle of mandible, except in the _Bathyerginæ_, rising from the inferior surface of the incisive alveolus. Clavicles well developed, except in _Lophiomys_. Tibia and fibula united.

_Family_ MYOXIDÆ.

Small arboreal forms, with long hairy tails, large eyes and ears, and short fore limbs. No cæcum in the intestine. Skull with narrow frontals, a high and narrow infraorbital vacuity of moderate size, and a long and slender coronoid process to the mandible. Premolars ¹⁄₁; molars rooted, with transverse enamel-folds.

The Dormice form a natural family allied to the Squirrels in form and habits, and confined to the Palæarctic and Ethiopian regions. The absence of the cæcum distinguishes them from all other members of the order. They are usually divided into the following five genera, but some of these are of very doubtful value, and it might be preferable to retain _Muscardinus_ and include all the others in _Myoxus_.[306]

_Myoxus._[307]—Represented by the European _M. glis_, and characterised by the bushy distichous tail, simple stomach, and the large size and complex enamel-folds of the molars, which have flat crowns.

_Eliomys._[308]—Tail tufted and distichous; stomach simple; and the molars small, with concave crowns and indistinct enamel-folds. Some seven species, Ethiopian and Palæarctic.

_Graphiurus._[309]—Tail short, cylindrical, and tufted at the end; molars very small, with the enamel-folds almost absent. Some three Ethiopian species.

_Claviglis._[310]—Represented by one West African species, said to be distinguished from all other forms by the shorter tail, which is more distinctly pencilled. The right to generic distinction is, however, very problematical.

_Muscardinus._[311]—Includes the Common Dormouse (_M. avellanarius_) of Europe, distinguished by the cylindrical bushy tail, and thickened glandular walls of the cardiac extremity of the œsophagus; the molars have flat crowns, with complex enamel folds.

_Fossil Dormice._—Using the generic term _Myoxus_ in a more extended sense than the above, it has existed in Europe from the date of the Upper Eocene. A species nearly as large as a Guinea-Pig, with very complex molars, is common in the Pleistocene of Malta.

_Family_ LOPHIOMYIDÆ.

The genus _Lophiomys_,[312] represented only by _L. imhausi_ (Fig. 204) of North-East Africa, differs from the typical _Muridæ_ in having the temporal fossæ roofed over by a thin plate of bone, rudimentary clavicles, and an opposable hallux. On these grounds it has been made the type of a family, but since all the features are Murine—the dentition being that of a typical Cricetine—it appears doubtful whether that distinction is justifiable. The hair forms a crest along on the back, and is of a peculiar structure. The habits of this Rodent are arboreal.

_Family_ MURIDÆ.

Skull (Fig. 203) with contracted frontals; a short and slender jugal, generally reduced to a splint between the zygomatic processes of the maxilla and squamosal; the lower root of the former process more or less flattened into a perpendicular plate; typically, the infraorbital vacuity tall, and wide above and narrow below. Lower incisors compressed; no premolars;[313] molars rooted, or rootless, tuberculate, or with angular enamel-folds. Pollex rudimental; tail generally nearly naked and scaly. Habits various, but mostly terrestrial.

This large and cosmopolitan family, which includes more than a third of the existing Rodents, is represented by about forty genera.

Subfamily =Hydromyinæ=.—Molars ²⁄₂ in number, rooted, and divided into transverse lobes. Represented by two Australasian genera.

_Hydromys._[314]—External form modified for an aquatic life. Tip of muzzle extensively haired, so that the nostrils can be closed. Skull with the infraorbital vacuity crescentic, scarcely narrowed below, and its external wall without the perpendicular zygomatic plate characteristic of most of the family; incisive foramina very small.

Two species, with habits like those of the Water Voles, are known from Australia, Tasmania, and New Guinea. In the typical _H. chrysogaster_ the colour of the back is black, with an admixture of golden-coloured hairs; the belly being of a dark golden hue.[315]

_Xeromys._[316]—External form Murine. Tip of muzzle as in _Mus_, not as in _Hydromys_. Toes unwebbed. Tail scaly, very finely haired. Skull as in _Mus_, with the exception of the rounding of the supraorbital edges. Teeth as in _Hydromys_.

Represented by _X. myoides_, of Queensland; a species about twice the size of the Common Mouse. This genus serves to connect _Hydromys_ with the other Murines, although it is difficult to say to which group it comes nearest.

Subfamily =Platacanthomyinæ=.—Molars rooted, with transverse laminæ. Flattened spines mingled with the hair; tail thickly haired. Represented by one genus.

_Platacanthomys._[317]—The one representative of this genus is _P. lasiurus_, found in the clefts of rocks and hollow trees in Southern India at elevations of about 3000 feet. This elegant little animal closely resembles a Dormouse; the tail and body having a length of 6 inches.

Subfamily =Gerbillinæ=.—Incisors narrow; molars with transverse laminæ (Fig. 205). Auditory bullæ very large in most cases. Hind limbs elongated. Tail usually long and hairy. Ranges over the Palæarctic, Oriental, and Ethiopian regions.

_Gerbillus._[318]—Upper incisors grooved; first molar with three laminæ, second with two, and third with one. There are some sixty species, with a range coextensive with that of the family. The Gerbils, with their large and bright eyes and long tufted tails, are very graceful creatures, inhabiting sandy plains, where they form extensive burrows. Remains of existing species are found in cavern-deposits in Madras (Fig. 205).

_Pachyuromys._[319]—The African genus _Pachyuromys_ is distinguished by the very large size of the auditory bulla, as well as by the short and fleshy tail, which is club-shaped. The incisors are narrow and faintly grooved.

_Mystromys_,[320] _Otomys_,[321] and _Dasymys_.[322]—These genera, also from South Africa, differ from _Gerbillus_ in the form of the molars, and are represented by a few species.

_Malacomys._[323]—The one known species of this genus is from the Gaboon, and is in some respect intermediate between the true Gerbils and the Rats. Thus the dentition and feet are those of the former, but the long scaly tail resembles that of the latter.

Subfamily =Phlœomyinæ=.[324]—This subfamily is represented only by _Phlœomys_[325] _cumingi_, of the Philippine Islands, in which the incisors are very broad, the molars are divided into transverse laminæ, and the claws are large. The muzzle is blunt; the ears are hairy externally; the tail is moderate, and thickly haired; and the auditory bullæ are very small. The first upper molar has three, and the others two laminæ.

Subfamily =Dendromyinæ=.—Incisors convex in front; molars ³⁄₃, rooted and tuberculated. Ears hairy; claws long. Confined to the Ethiopian region.

_Dendromys._[326]—A small Rodent, with the habits of a Dormouse, characterised by its grooved incisors, slender form, and long, scaly tail, which is sparsely haired. Two other Murines described as _Steatomys_[327] and _Lophuromys_[328] are referred to this subfamily. The first is of plump form, with a rather short and thickly haired tail, and grooved incisors. The latter resembles _Steatomys_ in form, but has fine flattened bristles instead of fur, and plain incisors.

Subfamily =Cricetinæ=.—Molars ²⁄₃, tuberculate and rooted, with the tubercles of the upper ones arranged in two longitudinal rows (Fig. 206, _B_). This subfamily has an almost cosmopolitan distribution, and appears to include the most generalised members of the family, from which the more specialised _Murinæ_ have been evolved.

_Cricetus._[329]—According to the arrangement proposed by Mr. O. Thomas[330] this genus is taken to include both the Hamsters of the Old World (_Cricetus_ proper) and the white-footed or Vesper Mice (_Hesperomys_) of the New. Cheek-pouches are frequently present, and may be very large. The first molar (Fig. 206, _B_) generally has six tubercles. The tail may be very short.

This large and unwieldy genus may be divided into a number of groups or subgenera. The typical group includes the Hamsters of the Old World, characterised by the large size of their cheek-pouches, the walls of which are connected with muscles arising from the lumbar vertebræ. The tail is remarkable for its shortness. The best-known species is _C. frumentarius_, inhabiting Europe and Northern Asia. The American forms, which range over the whole of that continent, comprise a number of subgenera, of which the following are the most important. _Rhipidomys_, including Dormouse-like forms with long tails and a dentition like that of the typical group; _Oryzomys_, represented by Murine species; _Calomys_, with short tail and Hamster-like body; _Vesperimus_, with only five tubercles on the first molar; _Onychomys_, in which the tail is extremely short and Hamster-like, and the form is Arvicoline; _Scapteromys_, of Murine form with a long and hairy tail; _Phyllotis_, with a shorter tail; _Habrothrix_, an Arvicoline group, with a short and thinly haired tail; and _Oxymycterus_, distinguished from the preceding by having a nail instead of a claw on the pollex. With regard to the distribution of these forms Mr. Thomas[331] remarks that in South America as we proceed southwards there is a general tendency “to a disappearance of the tropical and northern Mouse- and Dormouse-like subgenera _Rhipidomys_, _Vesperimus_, and _Oryzomys_, with the appearance and increase of the Vole- and Hamster-like _Habrothrix_ and _Calomys_—a change that is curiously paralleled in the Old World by the gradual supercession of _Mus_ and _Myoxus_ in favour of _Arvicola_ and _Cricetus_ as we go northwards from tropical to temperate and arctic regions.” One species has spines in the fur.

Remains of _Cricetus_ are abundant in the Pleistocene cavern-deposits of Brazil, where a number of the forms are referable to existing species; the genus is also represented in the Miocene of North America and Europe, the species from the former area having been described as _Eomys_, and those from the latter as _Cricetodon_.

_Holochilus_[332] (_Nectomys_).—The Rats of this genus are allied to the American forms of _Cricetus_, but have the third upper molars proportionately larger and the skull more stoutly built. This genus is confined to Brazil, and contains about six species, some of which are the largest indigenous Rats of America. Two species are aquatic in their habits, and have short webs between the toes of their hind feet.

_Sigmodon_[333] differs from _Cricetus_ in the pattern of the molar teeth. It contains one species only, the Rice-Rat, _S. hispidus_, ranging from the United States to Ecuador.

_Rhithrodon_,[334] and _Ochetodon_.[335]—These are more or less like _Cricetus_, but with grooved upper incisors. The first, is a South-American genus, and contains five Rat-like species, one from Venezuela, another from Peru, and the other three from Patagonia. The second consists of three North American mice, of about the size and proportions of the English Wood-Mouse (_Mus sylvaticus_).

_Neotoma._[336]—A peculiar North American genus, in which the teeth simulate the prismatic appearance of those of the _Arvicolinæ_. There are four species known as Wood-Rats, all of about the size of _Mus decumanus_; one of them (_N. cinerea_) having a tail almost as bushy as a Squirrel’s while the other three have ordinary scaly Rat-like tails.

Fossil remains of _Neotoma_ from cavern-deposits in Pennsylvania are not improbably referable to the existing Florida Rat (_N. floridana_). _Paciculus_, from the Miocene of the United States, is regarded as an allied extinct genus with enamel-folds to the molars.

_Hypogeomys._[337]—This and the following genera are confined to Madagascar, where they are the sole representatives of the Rodentia. _Hypogeomys_ is a very peculiar form of large size, with long ears, feet, and tail. There is only one species, _H. antimena_, a fawn-coloured Rat about 9 inches long.

_Nesomys._[338]—Contains two species of long-haired Rats, more or less rufous in colour, about the size of the Brown Rat.

_Brachytarsomys._[339]—Represented only by _B. albicauda_, a pretty velvety-haired fawn-coloured Rat, with short feet and a long tail.

_Hallomys._[340]—The only species (_H. audeberti_) is very like a _Nesomys_, but has much longer hind feet.

_Eliurus._[341]—Represented by one small Dormouse-like species, characterised by its nearly naked and short ears, and long tail, of which the proximal third is scaly, and the remainder covered with long hair. The pollex is rudimental, but the hallux well developed.

Subfamily =Arvicolinæ=.—Molars usually imperfectly rooted or rootless, and composed of two longitudinal rows of triangular prisms placed alternately (Fig. 207). Tail moderate or short. Common to the Palæarctic and Nearctic regions.

The Voles, as the members of this group are commonly termed, are so closely connected with the Cricetines that they may be regarded merely as a branch of that subfamily which has attained a peculiarly specialised type of molar dentition. The Voles are externally distinguished, as a rule, from true Rats and Mice by their more clumsy and heavy build and less graceful movements; by the small size of their eyes, the bluntness of the muzzle, the small ears, and the shorter limbs and tail.

_Phenacomys._[342]—A North American genus distinguished by its rooted molars, and thus connecting the typical forms with Cricetines like _Neotoma_. Several species have been described by Dr. C. H. Merriam.

_Arvicola._[343]—The type genus _Arvicola_ has rootless molars, and naked soles to the feet. It includes over forty species inhabiting Europe, North America, and Asia, a few species entering into the northern limits of the Oriental region in India. Three species of the genus are found in the British Isles, of which the following account is given by Mr. O. Thomas:—

The common Water-Vole (_A. amphibius_) is as large as the Brown Rat. Its fur is long, soft, and thick, of a uniform grizzled brown all over, except when, as is not uncommon, it is wholly black. The tail is about half the length of its head and body, and the hind feet are unusually long and powerful, although not webbed, and have five rounded pads on their lower surfaces. Its molar teeth (see Fig. 207) present the following number of prismatic spaces:—in the upper jaw the first, or anterior, has 5, the second 4, and the third 4, of which the last is very irregular in shape, and is sometimes itself divided into two, making 5 in all; in the lower jaw the first has 7 spaces, of which the 3 anterior are generally not fully separated from one another, the second has 5, and the third 3. These numbers for the different teeth are taken as the characters of the subgenus _Paludicola_ of Dr. Blasius, by whom this method of subdividing the genus was first introduced. The Water-Vole is one of the commonest English mammals, and is perhaps the most often actually seen of all, owing to its diurnal habits. It frequents rivers and streams, burrowing deeply into their banks, and in this way often causing considerable damage. Its food consists almost wholly of water-weeds, rushes, and other vegetable substances, but, like so many other Rodents, it will also occasionally eat animal food, in the shape of insects, mice, or young birds. The female during the warm season of the year has three or four litters, each of from two to seven young. The range of the Water-Vole extends over the whole of Europe and North Asia, from England to China, but it is not found in Ireland. The common Field-Vole, or short-tailed Field-Mouse (_A. agrestis_), representing the subgenus _Agricola_, is about the size of a House-Mouse, but with a short stumpy body, and a tail only about one third the length of the head and body combined. Its hind feet have six pads on their inferior surfaces. The colour is dull grizzled brown above, and grayish-white below. Its molar teeth have respectively 5, 5, and 6 prismatic spaces above, and 9, 5, and 3 below. The Field-Vole is one of the commonest of our smaller mammals, and frequents fields, woods, and gardens in enormous numbers, often doing very considerable damage in the latter, owing to its fondness for garden produce of all kinds. It is spread over the whole of Great Britain from the Hebrides southwards. Abroad its range extends from Finland to North Italy and from France and Spain to Russia. The Bank-Vole (_A. glareolus_) resembles in size and general appearance the common Field-Vole, but may be distinguished by its more or less rusty or rufous-coloured back, its larger ears, and the relatively longer tail, which attains to about half the length of the head and body. Its molar teeth present characters so different from those of all other Voles as to have caused it to be regarded as belonging to an entirely distinct genus, for which the name of _Evotomys_ has been used. Their chief distinction lies in the fact that, unlike those of all other Voles, their pulp-cavities close up in adult life, and they form distinct roots, more resembling those of the ordinary Rats and Mice. The enamel-spaces of these teeth number respectively 5, 4, and 5 above, and 7, 3, and 3 below. The habits of this species are in every way similar to those of the Field-Vole. Its range in Great Britain extends northwards to Morayshire, beyond which it has not yet been observed. It is also found all along the north temperate zone from France to China, and is replaced in North America by a closely allied animal known as _A. gapperi_. It is probable, however, that both _A. gapperi_ and _A. glareolus_ are only southern climatic offshoots of a still more northern species, the _A. rutilus_ of Northern Europe, Siberia, and Arctic America.

Fossil remains of _Arvicola_ are common in European Pleistocene deposits, and they have also been obtained from the Upper Pliocene of the Norwich Crag.

_Synaptomys._[344]—Represented by one North American species, having grooved upper incisors, skull and molars like those of _Myodes_, with the external characters of _Arvicola_.

_Myodes._[345]—Distinguished from _Arvicola_ by the more clumsy build, convex obtuse head, extremely short and Rabbit-like tail, short ears, small feet, the soles of which are furred, elongated claws, and thick fur, as well as by the breadth and massiveness of the skull, in which the zygomatic arch has a laminar expansion and the palate a peculiar contour; while the root of the lower incisor does not extend behind the last molar, the upper incisors are bevelled, and not grooved, and the molars have a characteristic pattern, which cannot be well explained without a figure.

The Lemmings, as the members of the genus are commonly called, are represented by the Norwegian Lemming (_M. lemmus_, Fig. 208), and the North American _M. obensis_. Different individuals of the Norwegian Lemming vary considerably both in size and colour, but its usual length is about 5 inches, and its soft fur yellowish brown, marked with spots of dark brown and black. It has a short, rounded head, obtuse muzzle, small bead-like eyes, and short rounded ears, nearly concealed by the fur. The tail is very short. The feet are small, each with five claws, those of the fore feet strongest, and fitted for scratching and digging. The usual dwelling place of the Lemmings is in the highlands or fells of the great central mountain chain of Norway and Sweden, from the southern branches of the Langfjeldene in Christiansand-stift to the North Cape and the Varangerfjord. South of the Arctic circle they are, under ordinary circumstances, exclusively confined to the plateaus covered with dwarf birch and juniper above the conifer region, though in Tromsö-amt and in Finmarken they occur in all suitable localities down to the level of the sea. The nest is formed under a tussock of grass or a stone, constructed of short dry straws, and usually lined with hair. The number of young in each nest is generally five, sometimes only three, but occasionally seven or eight, and at least two broods are produced annually. Their food is entirely vegetable, especially grass-roots and stalks, shoots of the dwarf birch, reindeer-lichens, and mosses, in search of which they form, in winter, long galleries through the turf or under the snow. They are restless, courageous, and pugnacious little animals. When suddenly disturbed, instead of trying to escape they will sit upright, with their back against a stone or other coign of vantage, hissing and showing fight in a very determined manner (Fig. 208).