WHILE the last few chapters have been devoted to orders which contain the largest and most powerful of terrestrial mammalia, we have now to treat of a group, all the members of which are of comparatively small size. “Mice, rats, and such small deer,” to use Shakspere’s phrase, make up a great proportion of the order Rodentia. The biggest of them is only about the size of a small Pig; and perhaps the common House Rat, or, at any rate, the common Squirrel, may be taken as showing the average dimensions of a Rodent. But, although from this point of view they may be looked upon as “a feeble folk,” their numerous species render them a most important section of the mammalian fauna of nearly all countries, and this importance is greatly increased, practically, by the immense number of individuals by which each species is usually represented.
The Rodentia, or gnawing mammals—GLIRES, as Linnæus and some modern zoologists call them—notwithstanding the great number of the species and the immense variety of forms which they display, constitute, perhaps, the most definitely circumscribed order of the Mammalia. In most other groups of the same value, we find that some types exhibit divergent characters, which render it difficult to frame a general description of the order which shall include them; or else some species present a marked tendency towards some other order; but in the case of the Rodents, we never have any difficulty, a cursory inspection of the dentition is always sufficient to decide whether a quadruped belongs to the Rodentia or not; and in spite of an almost infinite variety of form, the structure of the rest of the organism is most clearly in accordance with the evidence derived from the teeth.
SKULL OF THE TAGUAN, A FLYING SQUIRREL.
The teeth are only of two kinds—incisors and grinders (see the above figure of the skull of the Taguan)—and the number of efficient teeth of the former kind is never more than two in each jaw. Almost throughout the order, indeed, there are actually, even from the first, only two incisors present; but in the Hares and Rabbits, and some allied forms, there are in the upper jaw, in addition to the working teeth, a pair of rudimentary incisors,[48] placed immediately behind the large ones, but quite incapable of taking any part in the business of gnawing, for which the latter are so admirably fitted. Their presence is, however, of interest, as indicating the direction in which an alliance with other forms of Mammalia more abundantly supplied with teeth is to be sought.
DENTITION OF THE HARE.
The great incisors, which are characteristic of the Rodents, exhibit the following peculiarities:—They possess no roots, but spring from a permanent pulp, so that they continue growing during the whole life of the animal; and their form, and that of the cavity which constitutes their socket, is always that of a segment of a circle,[49] in consequence of which, they always protrude from the front of the jaws in the same direction, and meet at the same angle. By this means, as the teeth are worn away at their summits by use in gnawing, a fresh supply of tooth is continually being pushed forward to take the place of the portion thus removed, and, in fact, so intimately are the two functions of use and growth correlated in the teeth of these animals, that if by chance one of the incisors should get broken, or the natural opposition of these teeth should be disturbed in consequence of injury to the jaw, the teeth, thus deprived of their natural check, continue growing, and, following the curve of their sockets, gradually form circular tusks, which must always be greatly in the way of the animal when feeding, and sometimes, by actually penetrating again into the mouth, cause its death by absolute starvation. The teeth themselves are composed of dentine, coated along the front surface with a layer of hard enamel, which substance is wanting on the other surfaces of the teeth, except in the Hares, Rabbits, and other forms with additional rudimentary incisors in the upper jaw, in which, as further evidence of their relationship to the other Mammalia, the whole surface of the incisors is encased in enamel, although this coat is excessively thin except on the front or outer face. The purpose of this structure of the incisors is easily understood. In the action of gnawing, the dentine, which forms the greater part of the tooth, is more easily abraded than the harder enamel, which is thus left as a sharp front edge, to which the mass of dentine behind it, being worn away into a bevelled surface, gives the necessary firmness and support, the whole forming a chisel-like instrument, constructed precisely on the principle of those tools in which a thin plate of hard steel forms the cutting edge, and is stiffened by a thicker bevelled plate of softer iron.
The canine teeth are entirely deficient, and behind the incisors we find on each side a toothless gap of considerable extent (see figures p. 82), beyond which come the grinding teeth. In these it is difficult to recognise any distinction of molars and pre-molars; the whole series presents nearly the same structural characters, and for all practical purposes we may speak of them as molars, although some zoologists prefer to regard the three hindmost teeth on each side as true molars, and any others that may be present as premolars. In one genus (Hydromys) the number of grinding teeth is reduced to two on each side in each jaw; in a great proportion of the species the number is three; others have four or five grinders on each side, either in one or both jaws (usually one more in the upper series); and the largest number is possessed by the Hares and Rabbits, in which the upper jaw has six and the lower five grinders.[50] The grinders are sometimes furnished with true roots, but are more commonly open below, and provided, like the incisors, with a permanent pulp. They are sometimes tubercular, at least in youth, but generally show a flat, worn surface with transverse bands, or re-entering folds, and sometimes cylinders of enamel, which display a great variety of patterns. Sometimes the enamel is confined to the surface of the tooth; in other cases each tooth is, as it were, made up of two or more variously-shaped tubular portions of enamel, filled up with dentine. Curiously enough, this structure of the grinders, especially the arrangement of the transverse ridges and plates of enamel in these little animals, reminds us strongly of the characters of the molars of the gigantic Proboscidea, in which, moreover, the incisors also are represented by the permanently-growing tusks.
The articulation of the lower jaw with the skull is peculiar, and in special relation to the armature of teeth which we have described. Instead of articulating freely, as in man and many herbivorous mammals, by which provision is made for a sort of rotatory action of the molars, or by a regular transverse hinge-joint, as in the Carnivora, the articulating surfaces are elongated in a direction parallel to the middle line of the skull, an arrangement which, like that occurring in Carnivora, has the effect of preventing much lateral movement of the jaw; but, at the same time, the pits with which the jaw articulates are open in front, so that the jaw is allowed a certain amount of play, backwards and forwards. This motion greatly increases the gnawing power of the large incisor teeth.