As might be expected from the great number of species belonging to this order, and their general uniformity of structure, their classification is a matter of some difficulty, and very different views as to their relationships have prevailed at different times. Nowadays, however, zoologists have arrived at something like uniformity of opinion in this matter, and except in some minor points they may be said to be pretty nearly agreed. In the following sketch of the natural history of the Rodents we shall follow the classification proposed by the late Mr. E. R. Alston in the Proceedings of the Zoological Society. Mr. Alston accepted the division of the order into two primary groups (sub-orders), proposed fifty years ago by Professor Gervais, and characterised by the number of incisor teeth. The first of these sub-orders, which includes by far the majority of the Rodents, is formed by those species which never at any period of their lives possess more than two incisors in the upper jaw, and have the enamel on these strictly confined to the front surface of the teeth. These are denominated SIMPLICIDENTATA, or SIMPLE-TOOTHED RODENTS. In the second group, which includes only the Hares, Rabbits, and Calling Hares, we have those species which in the adult state possess four incisors in the upper jaw, namely, two large and efficient teeth, and behind these two small, almost rudimentary incisors (see figure of the dentition of the Hare on p. 82). These are called DOUBLE-TOOTHED RODENTS, or DUPLICIDENTATA.
SUB-ORDER I.—SIMPLE-TOOTHED RODENTS.
Besides the characters derived from the number of incisor teeth above mentioned, several other peculiarities of structure seem to show the existence of a decided difference between the Simple-toothed and Double-toothed Rodents; but most of these are of a rather abstruse nature, and need not be noticed here, the most important additional distinctive characters of the former being that the bony palate is well developed, and that the fibula does not articulate with the calcaneum, or heel-bone; whereas in the Double-toothed Rodents the palate is reduced to a mere bridge between the portions of the upper jaw in which the teeth are inserted, and the fibula does articulate with the heel-bone.
The Simplicidentata include a great number of families, and various attempts have been made to group these under larger heads; but it must be confessed that, owing to the way in which the families approach one another, it is difficult to bring them together in sections capable of being very strictly defined. Certain broad principles of relationship are, however, generally recognised, and Mr. Alston represented these by placing the Simple-toothed families under three great sections, the first indicated by Mr. Waterhouse—the Squirrel-like, Mouse-like, and Porcupine-like Rodents.
The Squirrel-like Rodents have four molars on each side in the lower jaw, and either four or five in the upper. When the latter number is present, the foremost tooth is smaller than the rest. The fibula remains as a distinct bone through life, and is usually quite free, although sometimes attached to the tibia at the extremity. The upper lip is usually cleft, the muffle is small and naked, and the nostrils are comma-shaped, with the rounded part above. The zygomatic arch is formed chiefly by the process of the malar bone, which is not supported below by a continuation of the zygomatic process of the maxillary. The collar-bones are perfect. The tail is usually cylindrical and hairy.
The Mouse-like Rodents agree with the preceding in the characters of the upper lip, muffle, and nostrils, but they have the tibia and fibula completely united for at least the last third of their length. The zygomatic arch is slender, and the malar process rarely extends so far forward as in the preceding group and is generally supported below by a continuation of the maxillary process. The collar-bones are perfect, except in one very small family; and the tail is cylindrical, and although sometimes hairy, more commonly covered with scales arranged in rings. The number of molar teeth in this section varies from three to six[51] on each side in each jaw, but three is the most usual number.
TEETH OF THE TAGUAN.
The Porcupine-like Rodents, with one exception, have four molars on each side in both jaws; the fibula distinct throughout life; the upper lip rarely cleft; the muffle clad with a velvety coat of fine hairs; and the nostrils either S-shaped or straight. The zygomatic arch is stout, and the malar process does not advance far forward, nor is it supported below by the maxillary process.