ORDER VIII. RODENTIA.
(Gnawing Animals.)
The Order before us, which is very extensive in genera and species, consists of animals, all of which may be characterized as of small, and, for the most part, of diminutive size. They may be readily known by the incisor teeth, which, two in number in each jaw, project from the very extremity of the jaw, in a forward curve: they are strong, compressed, and armed only on the
SKULL OF PORCUPINE.
front side with enamel; which being more durable than the bony matter of the rest, always maintains a sharp chisel-like edge to the teeth. They have no roots, but are deeply inserted in their sockets; and springing from a pulpy germ, are continually growing. ‘Thus provision is made to meet the loss by wearing down, which otherwise would very soon result from the use to which they are constantly applied: for the action of these teeth is not that of biting, as in the Apes, nor of tearing, as in the Cats, nor of cropping as in the Ruminants, but of gnawing, as the name Rodentia implies ; a patient scraping, filing, or nibbling away the surface of the substances which they attack, often the bark or wood of trees, or the shells of nuts, of almost a stony hardness. The hinge-joint of the lower-jaw is of such a character as to allow of no other horizontal motion than forwards and backwards; and hence the flat crowns of the molar teeth have their enamel ridges set transversely; so that the motion of the jaw brings their surfaces into more effectual contact for the purpose of grinding. In general the teeth are of a frugivorous character; but some have the molars set with blunt tubercles, or sharp points, and these are less restricted in their diet; the latter manifesting, indeed, more or less appetite for animal food. The canine teeth are always wanting.
“The structural organization of the Rodents,” observes Mr. Martin, ‘‘ as evidenced by the characters of the skull, the bird-like condition of the brain, and by other points, is at a low par, and the ratio of their intelligence is in a parallel degree. We may tame them, but we cannot educate them. They are all timid and feeble, and trust for self-protection to flight or concealment. The prey of ferocious beasts, and birds, and reptiles, their fertility, by a wise provision, counterbalances their annual diminution. Spread over the earth, from the equator to the coldest latitudes, they tenant rocks and mountains, plains and woods, feeding on grain and vegetables, and often devastating the cultivated domains of man . . . . . . Most are nocturnal, or crepuscular in their habits; many dwell in burrows; some conceal themselves amidst herbage, some amongst the foliage of trees, and some build for themselves habitations, which have excited the interest and admiration of man."[1]
We may consider the Rodentia as constituted of five Families, Sciuridæ, Muridæ, Castoridæ, Hystricidæ, and Leporidæ.
- ↑ Pict, Mus. i. 54.