CHAPTER XV. Rodentia--Tillodontia

RODENTIA--TILLODONTIA

ORDER IX. RODENTIA

Small to moderately large animals, furry, sometimes with spines. Toes with nails of a claw-like character, or sometimes approaching hoofs. Usually plantigrade, and only occasionally and partly carnivorous. Canine teeth absent; incisors long and strong, growing from persistent pulps, and with enamel only or chiefly on the anterior face, producing a chisel-shaped edge; molars few (two to six), separated from the incisors by a wide diastema. Caecum (nearly always present) very large, and often complicated in structure. Brain, if not smooth, with few furrows, the hemispheres not overlapping the cerebellum. Surface of skull rather flat; orbits not separated from temporal fossae; malar bone in middle of zygomatic arch; palate very narrow, with elongated incisive foramina; articular surface for lower jaw antero-posteriorly elongated. Clavicles generally present. Testes generally abdominal. Placenta deciduate, and discoidal in form.

The Rodents are a very large assemblage of usually small, sometimes quite minute, creatures, embracing an enormous number of living generic types. They are distributed all over the world, including the Australian region, and, being small and often nocturnal, and by no means particular in their diet, have managed to thrive and multiply to a greater extent than any other group of living mammals. They are chiefly terrestrial creatures, and often burrow or live in ready-made burrows. {459} Some, however, such as the Voles, are aquatic; others, e.g. the Squirrels, are arboreal, and there are "flying" Rodents exemplified by the genus Anomalurus. Their range of habitat is in fact as wide as that of any other Order of mammals, and wider than that of most.

FIG. 231.--Side view of skull of Cape Jumping Hare (Pedetes caffer). x 3/5. AS, Alisphenoid; Ex.O, exoccipital; Fr, frontal; L, lachrymal; Ma, malar; Mx, maxilla; Na, nasal; OS, orbito-sphenoid; Pa, parietal; Per points to the large supratympanic or mastoid bulla; PMx, premaxilla; Sq, squamosal; Ty, tympanic. (From Flower's Osteology.)

The most distinct anatomical characteristic of the Rodents concerns the teeth. They are without exception entirely deprived of canines. Thus there is a long diastema between the incisors and the molars. Another peculiarity is, that in many cases the dentition is absolutely monophyodont. In such forms as the Muridae there seems to be no milk dentition at all. In that family there are only three molars; but in other types where there are four, five, or six molars, the first one, two, or three, as the case may be, have milk predecessors, and may thus be termed premolars. This has been definitely proved to be the case in the common Rabbit, which has the unusually large number of six grinding teeth in each half of the upper jaw when adult. The first three of these have milk forerunners. On the other hand the existence of four molars does not apparently always argue that the first is a premolar; for Sir W. Flower found that in Hydrochoerus, none of the teeth had any forerunners, at any rate so far as could be detected from the examination of a very young animal. The Rabbit appears to be also exceptional, in that the second incisor of the upper jaw and the incisor of the lower jaw have milk forerunners. In any case the tendency towards monophyodontism is peculiarly well-marked in this group of mammals. The incisors of Rodents are as a rule in each jaw a single pair of long and strong teeth, which grow from persistent pulps, and {460} grow to a very great length, extending back within the jaw to near the hinder part of the skull. These teeth are reinforced in the upper jaw by a small second pair in the Lagomorpha only. The incisors are chisel-shaped, and often brown or yellow upon the outer face, as is the case also with some Insectivores. This peculiar shape, and their strength, renders them especially capable of the gnawing action which characterises the Rodents. It has been pointed out that where the incisors are wider than thick, the gnawing powers are feebly developed; and that on the contrary, where these teeth are thicker than wide, the animals are good gnawers. The incisors have often an anterior groove, or it may be grooves.

FIG. 232.--Molar teeth of Rodents. A, of Capybara (Hydrochoerus); B, of Squirrel (Sciurus); C, of Ctenodactylus. (After Tullberg.)

The cheek teeth vary in number from two (Hydromys) to six (Rabbit) on each side of the two jaws. Four is the prevailing number outside the large division of the Rat-like Rodents. They are often set at an angle to the horizontal plane of the jaw, looking outwards and obliquely to its longitudinal axis; the individual teeth too are not unfrequently bowed in form, reminding us of those of Toxodon. This of course only occurs in those genera which have hypselodont teeth. The pattern of the teeth varies much, and the different forms recall the teeth of more than one other group of mammals. They are either bunodont or lophodont. In many cases the tooth is encircled with a ridge of enamel, which is either almost simple or has a more complicated contour; such teeth are by no means unsuggestive of the Toxodonts. Some of the lophodont molars are by no means unlike those of the Proboscidea. In Sciurus vulgaris the {461} encircling ridge is broken up into tubercles, which gives to the tooth a striking likeness to those of Ornithorhynchus. Other genera have teeth like those of many Ungulates. It has been shown by Sir J. Tomes that the minute structure of the enamel differs in different groups of Rodents.

The skull shows certain primitive characters. In the first place there is no distinction between the orbital and the temporal fossa. The sutures between the bones retain their distinctness for very long. Other characteristic features are the following:--The nasals are large, and so are the paroccipital processes. The palate in front of the molars is not distinct from the sides of the skull, its edge gradually becoming rounded off above. It is also very narrow. The premaxillae are large in relation to the great incisors. There is often a very much enlarged infra-orbital foramen through which passes a part of the masseter muscle. The jugal bone lies in the middle of the zygomatic arch, which is complete and enormously enlarged in the Spotted Cavy (Coelogenys paca). As in many Marsupials, the jugal bone sometimes extends backwards to the glenoid cavity, where the lower jaw articulates. It is usually said with an absolute want of accuracy that the cerebral hemispheres of the Rodents are smooth and without convolutions. This error has been repeated again and again in text-books. As a matter of fact the cerebral hemispheres of many forms are quite well convoluted, the degree of furrowing corresponding, as in so many groups of mammals, with the size of the animal. This at any rate is generally true, though the large Beaver with its scant convolutions is an exception. The smaller forms, such as Mus, Sciurus, Dipus, and Cricetus are quite smooth-brained. The best furrowed brain of any Rodent which has been examined is that of the huge Hydrochoerus. The Sylvian fissure is very generally not pronounced; but is particularly well-marked in Lagostomus. In all, or in most, Rodents the hemispheres are separated by an interval from the cerebellum, the optic lobes being visible between the two.

The mouth cavity of this group of mammals is divided into two chambers by a hairy ingrowth behind the incisors; this arrangement is useful for animals which use their strong incisors {462} as gnawing and excavating tools as well as for the purposes of alimentation; for it allows of substances being gnawed away without the products of the chisel-like action being taken into the hinder cavity of the mouth. The Rodents have for the most part a simple stomach of normal form; but in a few this is complicated by a marked constriction, which divides the cardiac from the pyloric portions. The Hamster, for example, is thus characterised. In all the members of the order, with the exception of the Dormice and some allied forms, the caecum is large and often sacculated. In some forms (e.g. Arvicola, Myodes, Cuniculus) the large intestine is coiled upon itself in a spiral way--a state of affairs strongly suggestive of Ruminants.

The Rodents are primarily divisible into two great groups, the Simplicidentata and the Duplicidentata, characterised mainly by the upper incisor teeth. In the former there is but one pair of these teeth; in the latter a second smaller pair lie behind the former.

SUB-ORDER 1. SIMPLICIDENTATA.