[ORDER RODENTIA.]
THE GNAWERS.
This order, GLIRES of Linnæus and his followers, is composed of animals, chiefly of small size, which differ from all others by the peculiarity of their teeth. No one, even though he be most ignorant of comparative anatomy, could mistake the rat or rabbit-like skull of a rodent for that of any other creature. The peculiar pincer-like form of the jaws, with their curved chisel-shaped teeth in front, mark the order at a glance. There is no complexity in their dentition. There are the cutters or incisors, and the grinders; and of the cutters there are never more than two in each jaw, that is to say efficient and visible teeth, for there are in some species rudimentary incisors, especially in the young, but these either disappear or take no part in work. Between the grinders and incisors are toothless gaps. The formation and growth of the teeth are peculiar; and it is strange that the gigantic elephant should be the nearest approach to these small creatures in this respect. The teeth—in most cases the grinders, but always the incisors—grow continuously from a persistent pulp, and therefore loss from attrition is kept constantly supplied by growth from behind. The incisors are planted in a socket which is the segment of a circle. These segments are not equal in both jaws. The lower one is a small segment of a large circle, the upper one is the reverse, being a larger segment of a smaller circle. The angle at which they meet is always the same. Some curious malformations are occasionally found which illustrate the growth of these teeth. Should by any chance, accident or design, one of these incisors get diverted from its proper angle and not meet with the friction which is necessary to keep it in its normal condition, it goes on growing and growing, following its natural curve till it forms a ring, or by penetrating the mouth interferes with the animal's feeding. A case is recorded by Blyth of a rat which had an eye destroyed by a tooth growing into it. Here again occurs a similarity to the elephant, whose tusks grow in the same manner, and if abnormally deflected will occasion, as in the case of one lately described to me, serious hindrance to the movement of the trunk. The incisors of rodents are composed of dentine coated in front with a layer of hard enamel, the other surfaces being without this protection, except in the case of some, amongst which are the hares and rabbits, which have a thin coating as well all over. These forms are those with rudimentary incisors, and constitute the links connecting the other mammalia with the Gnawers.
The molars are much alike in structure, and can hardly be divided, as they are by some naturalists, into molars and premolars. They take the three hindmost as molars, regarding the others as premolars. Sometimes these grinders have roots, but are more commonly open at the end and grow from a permanent pulp. They are composed of tubular and convoluted portions of enamel filed up with dentine, and their worn surfaces show a variety of patterns, as in the case of the Proboscidea. These enamelled eminences are always transverse, and according to Cuvier those genera in which these eminences are simple lines, and the crown is very flat, are more exclusively frugivorous; others, in which the teeth are divided into blunt tubercles, are omnivorous; whilst some few, which have no points, more readily attack either animals, and approximate somewhat to the Carnivora.
The head is small in proportion to the body, the skull being long and flat above; the nasal bones are elongated; the premaxillaries very large on account of the size of the incisor teeth, and the maxillaries are, therefore, pushed back; the zygomatic arch is well developed in most, but is in general weak; the orbit of the eye is never closed behind; the tympanic bulla is very large; the jaw is articulated in a singular manner; instead of the lateral and semi-rotary action of the Herbivora, or the vertical cutting one of the flesh-eating mammals, the rodent has a longitudinal motion given by the arrangement of the lower jaw, the condyle of which is not transverse, but parallel with the median line of the skull, and the glenoid fossa, or cavity into which it fits, and which is situated on the under side of the posterior root of the zygoma, is so open in front as to allow of a backwards and forwards sliding action. The vertebral column is remarkable for the great transverse processes directed downwards, forwards, and widening at the ends. In the hare these processes are largely developed; the metapophyses or larger projections on each side of the central spinous process are very long, projecting upwards and forwards; the anapophyses or smaller projection in rear of the above are small; and the hypapophyses or downward processes are remarkably long, single and compressed; according to Professor Flower these latter are not found in the Rodentia generally. The tail varies greatly, being in some very small indeed, whilst in others it exceeds the length of the body; the sternum or breast-bone is narrow and long, and collar-bones are to be found in most of the genera; the pelvis is long and narrow. In most cases the hind limbs are longer and more powerful than the fore-limbs; in some, as in the jerboas (Dipus) and the Cape jumping hare (Pedetes caffer), attaining as disproportionate a length as in the kangaroos, their mode of progression being the same; the tibia and fibula are anchylosed; the forelimbs in the majority of this order are short, and are used as hands in holding the food to the mouth, the radius and ulna being distinct, and capable of rotatory motion. The feet have usually five toes, but in some the hind feet have only four, and even three. In point of intelligence, the rodents do not come up to other mammals, being as a rule timid and stupid; the brain is small and remarkably free from convolution. The cerebellum is distinctly separated from and not overlapped by the hemispheres of the cerebrum; the organs of smell, sight and hearing are usually well developed; the stomach is simple or in two sacs; the intestinal canal and cæcum long. The latter is wanting in one family.
Rodents have been divided in various ways by different authors. Jerdon separates his into four groups, viz. "Sciuridæ, squirrels; Muridæ, rats; Hystricidæ, porcupines; and Leporidæ, hares; which indeed are considered by some to embrace the whole of the order; to which has recently been added the Saccomyidæ, or pouched rats, whilst many systematists make separate families of the dormice, Myoxidæ; jerboas, Dipodidæ; voles, Arvidolidæ; mole-rats, Aspalacidæ and Bathyergidæ; all included in the MURIDÆ; and the Caviadæ, Octodontidæ, and Hydrochoeridæ, belonging to the HYSTRICIDÆ" ('Mammals of India,' p. 164).
However, the system that most commends itself is that of Mr. E. R. Alston, proposed in the 'Proceedings' of the Zoological Society, and founded on the original scheme of Professor Gervais, by which the order is subdivided into two on the character of the incisor teeth. Those which have never more than two incisors, coated only in front with enamel are termed SIMPLICIDENTATA, or Simple-toothed Rodents. The other sub-order, the genera of which have rudimentary incisors, as in the case of hares, rabbits, &c., and in which the enamel is spread more or less over all the surface, is termed DUPLICIDENTATA or Double-toothed Rodents, and this is the system I propose to follow.
SIMPLE-TOOTHED RODENTS.