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Originally appearing in Volume V23, Page 441 of the 1911 Encyclopedia Britannica.
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RODENTIA, or. GLIRES, an order of placental mammals characterized by the peculiar form and structure of their front or incisor teeth, which are reduced to a single functional chisel-like pair in each jaw, specially adapted for gnawing, and growing throughout the entire life of their owners. Rodents may be characterized as terrestrial, or in some cases arboreal or aquatic, placental mammals of small or medium size, with a milk and a permanent series of teeth, plantigrade or partially plantigrade, and generally five-toed, clawed (rarely nailed or semi- hoofed) feet, clavicles or collar-bones (occasionally imperfect or rudimentary), no canine teeth, and a single pair of lower incisors, opposed by only one similar and functional pair in the upper jaw. In all rodents the upper incisors resemble the lower ones in growing uninterruptedly from persistent pulps, and (except in the hare group, 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 are always open behind, never being surrounded by bone. The condyle of the lower jaw is antero-posteriorly elongated. The intestine (except in the dormice or Gliridae) has a large caecum. The testes are inguinal or abdominal. The uterus is two-horned, with the cornua opening separately into the vagina or uniting to form a corpus uteri. The placenta is discoidal and deciduate. And the smooth hemispheres of the brain do not extend backwards so as to cover any part of the cerebellum. Rodents include by far the greater 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, as in South America, which may be considered their headquarters, than in others, as in Australasia and Madagascar, where they are represented only by members of the mouse-group, or Myoidea. All rodents are vegetable-feeders, and this uniformity in their food and in the mode of obtaining it, namely by gnawing, has led to that general uniformity in structure observable throughout the group; a feature which renders their classification difficult. Indeed, despite the fact that they present much diversity of habit—some being arboreal, as the squirrels, many of which are provided with expansions of skin or parachutes on which they glide from tree to tree; some cursorial, as the hares; others jumpers, as the jerboas; others fossorial, as the mole-rats; and others aquatic, as the beavers and waterrats—no important structural modifications are correlated with such diversity of habit. Anatomy.—The rodent skull is characterized by the great size of the premaxillae, which completely separate the nasals from the maxillae; by the presence of zygomatic arches; and by the wide unoccupied space existing between the incisors and the cheek-teeth ; and (except in the Duplicidentata) by the antero-posteriorly elongated glenoid cavity for the articulation of the lower jaw. Post-orbital processes of the frontals exist in squirrels, marmots and hares; but in all other genera they are rudimentary or altogether absent; and the zygoma seldom sends upwards a corresponding process, so that the orbit is more or less completely continuous with the temporal fossa. The lachrymal is always within the orbital margin ; and in many species the infra-orbital fora-men is very large (in some as large as the orbit) and transmits part of the masseter muscle. The zygomatic arch is variously developed, and the position of the fugal is a character for grouping the families. The nasals are, with few exceptions, large, and extend far forwards, the pari- etals are moderate, and there is generally a distinct interparietal. The palate is narrow from before backwards, this being especially the case in the hares, where it is reduced to a mere bridge between the premolars; in others, as in the rodent-moles (Bathyerginae), it is extremely narrow transversely, its width being less than that of one of the molar teeth. Tympanic bullae are always present and generally large ; in some genera, as in the gerbils (Gerbillinae) and jerboas (Jaculidae), there are supplemental mastoid bullae which form great hemispherical bony swellings at the back of the skull (fig. I, Per),. in these genera and the hares the meatus auditorius being tubular and directed upwards and backwards. The lower jaw is characterized by its abruptly narrowed and rounded front part supporting the pair of large in- cisors, as well as by the small size of the coronoid process, and the great development of the lower hind, or angular, portion. The dental formula varies from i. c. $, P. $ , vs. 1 (total 28) in the hares and rabbits to i. 1, c. $, p. $, m. i (total 12) in the Australian water-rats; but in the great majority of species it presents striking uniformity, and may be set down typically as i. 1, c.$,p.1ore,m. . In the Duplicidentata only is there more than a single pair of incisors, and in these the additional pair is small and placed behind the middle pair. In this group the enamel extends partially to the back of the incisors, but in all the rest it is restricted to the front surface, so that, by the more rapid wearing-away of the softer structures behind, a chisel-shaped edge is maintained. Both upper and lower incisors are regularly curved, the upper ones slightly more so than the lower; and, their growth being continuous, should anything prevent the normal wear by which their length is regulated—as by the loss of one of them, or by displacement owing to a broken jaw or other cause—the unopposed incisor may gradually curve upon itself until a complete circle or more has been formed, the -tooth sometimes passing through some part of the animal's head. The cheek-teeth may be either rooted or rootless, and either cusped or formed of parallel plates, this diversity of structure often occurring in the same family. When there are more than three cheek-teeth, those which precede the last three have succeeded milk-teeth, and are premolars. In some species, as in the agoutis (Dasyproctidae), the milk-teeth are long retained, while in the allied cavies (Caviidae) they are shed before birth. The tongue presents little variability in length, being short and compressed, with a blunt tip, which is never protruded beyond the incisors. In most species there are three circumvallate papillae at the base, and the apical portion is generally covered with small thread-like papillae, some of which in the porcupines become greatly enlarged, forming toothed spines. The stomach varies in form from the simple oval bag of the squirrels to the complex ruminant-like organ of the lemmings. In the water-rat and agoutis it is constricted between the oesophagus and pylorus; while in the dormouse the oesophagus immediately before entering the stomach is much dilated, forming a large egg-shaped bag with thickened glandular walls; and in certain other species, as in Lophiomys and the beaver, glandular masses are attached to and open into the cardiac or pyloric pouches. All rodents, with the sole exception of the dormice, have a caecum, often of great length and sacculated, as in hares, the water-rat and porcupines; and the long colon in some, as the hamster and water-rat, is spirally twisted upon itself near the commencement. The liver is divided in the typical manner in all, but the lobes are variously subdivided in 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) may be more or less completely retracted within the fold of integument surrounding the vent, and lie curved backwards upon itself under cover of the integument, or it may be carried forward some distance in front of the anal orifice, from which, as in voles and marmots, szo Flower, Osteol. Mammal. m in the breeding-season, it is separated by the prominent testicular mass. The testes in the pairing-season form projections in the groins, but (except in the Duplicidentata) do not completely leave the cavity of the abdomen. Prostate glands and, except in the Duplicidentata, vesiculae seminales are present in all. The uterus may be double, each division opening by a separate os uteri into a common vagina, as in Leporidae, Sciuridae, and Hydrochoerus, or two-horned, as in most species. The teats vary in number from a single abdominal pair in the guinea-pig to six thoracico-abdominal pairs in the rats; while in the Octodontidae and Capromyidae they are placed high up on the sides of the body. - There are generally nineteen dorso-lumbar vertebrae (thirteen thoracic and six lumbar), the form of which varies in different genera ; in the cursorial and leaping species the lumbar transverse processes are generally very long, and in the hares there are large compressed inferior spines, or hypapophyses. The caudal vertebrae vary from a rudimentary condition in the guinea-pig to a great size in the jumping-hare and prehensile-tailed porcupines. The scapula is usually narrow, with a long acromion ; the clavicles may be altogether absent or imperfect, as in porcupines, cavies and hares, but in most species are well developed. The humerus has no supra-condylar foramen, and the forearm bones are distinct; and in most species the fore foot has five digits with the phalanges normally developed, the first toe being but rarely rudimentary or absent. The pelvis has large ischia and pubes, with a long and usually bony symphysis. The femur varies considerably in form, but generally has a well--defined third trochanter. In the squirrels and porcupines the tibia and fibula are distinct, but in rats and hares they are united, often high up. The hind foot is more variable than the front one, the digits varying in number from five, as in squirrels and rats, to four, as in hares, or even three, as in the capybara, viscacha and agouti. In the Jaculidae the metatarsals are greatly elongated, and in some of the species, as jerboas, they are welded together. The mouth is divided into two cavities communicating by a narrow orifice, the anterior one containing the incisors and the posterior the molars, the hairy skin of the face being continued inwards behind the incisors. This evidently prevents substances not intended for food getting into the mouth, as when the animal is engaged in gnawing through an obstacle. In hares and pacas the inside of the cheeks is hairy; and in some species, pouched rats and hamsters, there are large internal cheek-pouches lined with hair, which open near the angles of the mouth and extend backwards behind the ears. In the New World pouched rats (Geomyidae) the pouches open externally on the cheeks. The peculiar odour evolved by many rodents is due to the secretions of special glands, which may open into the prepuce, as in Mus, Microtus and Cricetus, or into the rectum, as in Arctomys and Thryonomys, or into the passage common to both, as in the beaver, or into pouches opening near the vent, as in hares, agoutis and jerboas. The skin is generally thin, and the panniculus carnosus muscle rarely much developed. The fur varies exceedingly in character,—in some, like the chinchillas and hares, being fine and soft, while in others it is more or less replaced by spines on the upper surface, as in spiny rats and porcupines; these spines in several genera, as Xerus, Acomys, Platacanthomys, Echinothrix, Loncheres and Echinomys, being flattened. In muscular structure the chief peculiarities are noticeable in the comparatively small size of the temporal muscles, and in the great double masseters (fig. 2), which are the principal agents in gnawing. The digastric muscles also are remarkable for their well-defined central tendon, and in many species their anterior bellies are united between the two halves of the lower jaw. The cleido-mastoid generally arises from the basi-occipital, and the pectoralis major is connected with the latissimus dorsi. In porcupines and hares the tendons of the flexor digitorum longus and flexor hallncis 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 first toe. Classification.—Some diversity of view obtains among naturalists with regard to the classification of the order; the scheme here followed being the one adopted (with some modifications of nomenclature) by Professor Max Weber in his Sdugethiere. The number of genera is so great that only the more important can be noticed. All authorities are agreed in dividing rodents into two great sections or sub-orders, the one, Duplicidentata, comprising only the hares, rabbits and picas, and the other, Simplicidentata, all the rest. In the latter there is only one pair of incisor teeth in the upper jaw, in which the enamel is confined to the front surface. The incisive foramina of the palate are moderate and distinct; the fibula does not articulate with the calcaneum; and the testes are abdominal, and descend periodically only into the inguinal canal. Sewellels.—The first family is represented by certain peculiar North American rodents known as sewellels, constituting the genus Haplodon (or Aplodon) and the family Haplodontidae and section Haplodontoidea. In common with the next three sections these rodents have the angular process of the lower jaw (fig. 4) arising from the inferior surface of the socket of the incisor, The masseter muscle does not pass through the narrow infra-orbital canal. An alisphenoid canal may be present on the palatal aspect of the skull; but there is always a transverse canal. The malleus and incus of the inner ear are separate. The humerus often has a foramen (entepicondylar) on the inner side of its lower end; the tibia and fibula may be separate or united; but the scaphoid and lunar of the carpus are also united, while the centrale is free. The stomach is simple. Sewellels are medium-sized terrestrial rodents, with no post-orbital process to the skull, which is depressed in form, and root-less cheek-teeth, among which the premolars number y, the first in the upper jaw being very small. The build is stout and heavy, the limbs and tail are short, the ears moderate, the eyes minute and the feet five-toed and plantigrade. Haplodon is represented by a small number of species in America west of the Rocky Mountains, of which H. rufus is the longest known. They are burrowing, and, in some cases at any rate, partially aquatic rodents. Squirrel Group.—The Sciuroidea, which include the great group of squirrels, sousliks, marmots, &c., all comprised in the single family Sciuridae, differ from the sewellels in having large post-orbital processes to the skull (figs. 4, 5, 6) ; and, with one exception, have rooted cheek-teeth, the premolar-formula being L''-El The infra-orbital foramen is also narrower, ' and the tympanic bulla is cellular. In both groups the tibia and fibula are separate. The family is divided into three sub-families, the first of which is the Sciurinae. In this the crowns of the molars are more or less shortened, with their cusps either arranged in longitudinal lines, or forming four upper and three lower more or less distinct oblique ridges. The post-orbital processes of the frontal and jugal are widely sundered, and the former may even be small (Xerus). The expanded anterior root of the zygomatic process has its front border oblique. According to modern views the sub-family is broken up into a large number of genera. The first of these is Rhithrosciurus, represented by one large species (R. notatus) from Borneo, characterized by its finely grooved incisors (see GROOVE-TOOTHED SQUIRREL). The second genus, Heliosciurus, includes arboreal African squirrels, typified by H. stangeri, allied in the characters of their skulls to the under-mentioned Xerus, and with a very large pre-orbital foramen in the more typical forms. The third, Funisciurus, of which F. pyrrhopus is a well-known example, is also African and allied to Xerus, but has a still longer skull and soft fur. In Xerus itself, which is represented by the terrestrial African spiny squirrels, the ears are short, there are only two teats, and flat spines are mingled with the fur; while the skull, and more especially the frontals, is elongated, with a very short post-orbital process, and the crowns of the molars are taller than usual (see SPINY SQUIRREL). The well-known Indian palm-squirrel, Funambulus palmarum, typifies an Indo-Malay genus allied to Xerus in skull-characters but with molars more like those of Sciurus. In contrast to these small striped species are the giant squirrels of the same region, such as Ratufa indica and R. bicolor, which are very brightly coloured rodents, with Sciurus-like skulls (fig. 5) but extremely short-crowned molars, and only one pair of upper premolars. Next comes the typical Sciurus, including the great bulk of the entire group, and ranging over Europe, Asia, North Africa and America. The skull is short and broad, especially as regards the frontals, with large post-orbital processes (fig. 5), and very generally two upper premolars, making a total of five pairs of upper cheek-teeth, which have crowns of medium height. The teats are either four or six. Squirrels of this and the other arboreal groups have the bodily form slender and agile, the tail long and bushy, the ears well developed, pointed and often tufted; the feet adapted for climbing, the anterior pair with four toes and a rudimentary thumb, and the posterior pair with five toes, all the toes having long, curved and short-pointed claws (see SQUIRREL). The names Glypholes and Sciurotamias have been proposed respectively for one Bornean and some four Chinese squirrels. With Tamias (sometimes split into Tamias and Eutamias) we reach the North American striped ground-squirrels, or chipmunks, well characterized by the large internal cheek-pouches, with one outlying species in Northern Asia and Europe (see GROUND-SQUIRREL). These lead on to the sousliks, Spermophilus (or Citellus), in which the incisors (as in the following genera) differ from those of all the squirrels in not being compressed. The genus which is common to the northern parts of both hemispheres is distinguished by the large cheek-pouches and by the absence or rudimentary condition of the claw of the first hind-toe, resembles Tamias in the slender form of the body, but displays great variation in the length of the tail, which may be a mere stump, or comparatively long. As in the following genera, there are two pairs of premolars, of which the first in this case is small and rounded, while the two series of cheek-teeth are nearly parallel (see SOUSLIK). The prairie-dogs, or prairie-marmots, Cynomys, are a North American group, in which the five-toed forefeet have the claw of the first as large as that of the fifth toe. The skull is heavily built, with the post-orbital processes directed outwards. Dentition (fig. 6) remarkably heavy, the molar teeth differing from those of Spermophilus and Arctomys by having three instead of two transverse grooves on their crowns. First premolar nearly as large as the second. Molar series strongly convergent behind (see PRAIRIE-MARMOT). Finally, we have the marmots (Arctomys), which are larger and more heavily built rodents, with short ears, more or less short tails and rudimentary or no cheek-pouches. Fore-feet with the first toe rudimentary and bearing a flat nail. Skull (fig. 4) large and heavy, with the post-orbital process stouter 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 genus is common to the northern half of both hemispheres, and its members, like those of the two preceding groups, burrow and hibernate (see MARMOT). The Nannosciurinae, or second sub-family of Sciuridae, are represented only by the pigmy squirrels (Nannosciurus), characterized by their very short-crowned molars (which approximate to those of dormice in structure) and small premolars, of which the first upper pair is often deciduous, while the upper molars have only three oblique ridges. The front root of the zygomatic arch is nearly vertical, and placed so far back that it is above the second molar, while the orbit—a unique feature among rodents—is almost completely surrounded by bone. The few representatives of this group are all very small rodents, confined to tropical Africa, the Philip-pines and the Malay islands. The third and last sub-family, the Pteromyinae, is distinguished from the other two by the presence of a parachute-like fold of skin along the sides of the body, the supporting cartilage of which arises from the carpus or wrist. It includes Sciuropterus, represented by small species from the northern parts of both hemispheres; Pterornys, comprising large flying-squirrels, ranging from India and the Malay countries to Japan, characterized by the long cylindrical tail and large inter-femoral membrane; and Eupetaurus, represented by one very large dark grey, long-tailed and long-haired species from Astor and Gilgit, which differs from all other members of the family by its tall-crowned cheek-teeth (see FLYING-SQUIRREL). Beavers.—The second section, Castoroidea, of the present group includes only the family Castoridae, represented by the beavers, which are large aquatic rodents characterized by their massive skulls, devo;el of post-orbital processes, with the angle of the lower jaw rounded, the molars rootless or semi-rooted, with re-entering enamel-folds, and, one pair of premolars above and below. The tibia and fibula are united inferiorly, the tympanic bulla is hollow and the infra orbital foramen narrow. The single existing genus comprises the European beaver, Castor fiber, of Europe and Northern Asia, and the North American C. canadensis. 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 oesophageal orifice; the anal and urino-genital orifices open within a common cloaca; the tail is broad, horizon-tally flattened and naked; and the hind-feet are webbed (see BEAVER). Pouched Rats.—The American pouched rats, or pocket-gophers, constitute the third section, Geomyoidea, with the single family Geomyidae. The dentition includes one pair of premolars above and below, and rooted or rootless molars with but few enamel-folds. In the skull the infra-orbital foramen is narrow, and post-orbital processes and an alisphenoid canal are absent. The tibia are fibula are united. The cheeks are provided with large pouches opening externally. Two sub-families are recognized. The first of these, or Geomyinae, is characterized as follows: Incisors broad; mastoid not appearing on the top of the skull; eyes small; ears rudimentary; limbs short, subequal. Habits fossorial. G'eomys bursarius, the " red pocket-gopher " of North America, with deeply grooved incisors, inhabits the plains of the Mississippi, living in burrows like the mole. Several other species from the Southern States, Mexico and Central America are recognized. Thomomys talpoides, with plain incisors, extending from Canada to the United States west of the Rocky Mountains, typifies the second genus, which has also many species. The following are the characters of the second sub-family, Heteromyinae: Incisors narrow; mastoid appearing largely on the top of the skull ; eyes and ears moderate or large; hind-limbs and tail elongated. . Habits terrestrial. Dipodomys, which has the molars rootless, is typified by D. phillipi, the kangaroo-rat of the desert regions east of the Rocky Mountains, Perodipus and Microdipodops being allied genera. Perognathus and Heteromys have rooted molars; the latter genus is distinguished by the presence of flattened spines among the fur, and has species extending into South America. (See POCKET-GOPHER, POCKET-MOUSE and KANGAROO-RAT.) Scaly-tailed Squirrels.—The next section, according to Prof. Max Weber's arrangement, is that of the Anomaluroidea, typified by the rodents commonly called African flying-squirrels (Anomaluridae), but better designated scale-tailed squirrels, or simply " scaly-tails," since one member of the family has no parachute. To this group Prof. H. Winge affiliates the African jumping-hares (Pedetidae), a view which is adopted by Prof. Weber, although Mr O. Thomas places these rodents in the neighbourhood of the porcupines. In the more extended sense, the Anomaluroidea are diagnosed as follows: In the skull the infra-orbital foramen (or canal) is large, the lachrymal foramen placed high up, and no transverse canal; while the malleus and incus of the internal ear are fused. In the carpus the scaphoid and lunar bones are united. There is a single pair of premolars in each jaw. The Anomaluridae are characterized by having rooted cheek-teeth with shallow transverse enamel-folds, the two halves of the lower jaw movably articulated in front, very small post-orbital processes to the skull, and the presence of two rows of scales on the under surface of the base of the tail (figs. 7 and 8), which is cylindrical and thickly haired., The family is confined to the equatorial forest-tract of Africa, where it is most numerously represented on the west side. The majority of the species belong to the typical genus Anomalurus (fig. 7), which is provided with a parachute supported by a cartilaginous process arising from the olecranon of the ulna, and has well-developed ears and a moderately long tail. Several of the species are considerably larger than an ordinary squirrel. Idiurus, as represented by the West African I. zenkeri (figured in the article FLYING-SQUIRREL), iS a mouse-like form, with very small ears and an extremely long tail. The third genus, Zenkerella (Aethurus), which is also West African, has no parachute (fig. 8). Jumping-Hares.—The grounds for referring the African jumping-hares (Pedetidae) to the Anomaluroidea rest largely on the evidence of certain Tertiary rodents from Europe, such as Issiodoromys. The family is represented by the South African Pedetes caller, which is as large as a hare, and the smaller East African P. surd aster. In general habits and appearance these animals recall large jerboas, from which group they are, however, distinguished by the four pairs of rooted cheek-teeth, the premolars being as large as the molars, and the latter having one outer and one inner enamel-fold. The hind-limbs are elongated, with four toes, of which the meta-tarsals are separate; the tibia and fibula are welded in old age; the calcaneum and astragalus of the tarsus are elongated; and there is a perforation on the inner side of the lower end of the humerus (see JUMPING-HARE). Dormice.—The next three sections of the order, namely, the Myoxoidea, or dormice, Dipodoidea, or jerboas, and Myoidea, or the mouse group, have the following characteristics in common. The From Alston. angular process of the lower jaw has the same relations as in the sewellels and the allied groups. The lachrymal foramen in the skull is low down and forms an elongated slit. In the carpus the sca- phoid and lunar are welded, but the centrale remains distinct. The tibia and fibula are fused at their upper and lower ends. The malleus and incus of the inner ear are separate. Except in Lophiomys, the clavicles are complete. The infra-orbital foramen of the skull (fig. 9) is more or less broad; and there is generally a transverse canal. The stomach is generally complex. In the dormice, forming the section Myoxidea, with the single family Gliridae (or Myoxidae), a single pair of premolars may or may not be present; the molars are short-crowned and rooted, with transverse From de Winton. enamel-folds. The angle (Zenkerella insignis). and its coronoid process slender. Dormice a r e small arboreal rodents, with long hairy tails, large eyes and ears, and short fore-limbs, ranging over Europe, Asia and Africa. Of the four genera in the typical sub-family Glirinae, the first is Glis, represented by Glis vulgaris (or G. glis) of Europe, with a doubly vaned, bushy tail, simple stomach, and large molars with well-marked enamel-folds; the second, Muscardinus, with M. avellanarius, the common dormouse, distinguished by the cylindrical bushy tail, and thickened glandular walls of the cardiac extremity of the oesophagus; thirdly, Eliomys, containing several species, with tufted and doubly vaned tails, simple stomachs and smaller molar teeth, having cgecave crowns and faintly marked enamel-folds; and lastly, the African Graphiurus, represented by several species, with short cylindrical tails ending in a pencil of hairs, and very small molars almost without trace of enamel-folds. None of the members of the typical sub-family extend into India, where the group is represented by Platacanthomys, typifying the sub-family Platacanthomyinae, characterized by the absence of premolars; the other being the Chinese Typhiomys. These are small rodents with somewhat the appearance of the pigmy squirrels (Nannosciurus), which in some degree connect the family with the Muridae. (See
End of Article: RODENTIA

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