These, as I before observed, are those of the order which never have more than two incisors in the upper jaw, and the enamel on these is restricted to the front of the tooth. They have also a well-developed bony palate, which in the Duplicidentata is imperfect, forming in fact but a narrow bridge from one jaw to the other. In the latter also the fibula, which is anchylosed to the end of the tibia, articulates with the calcaneum or heel-bone, which is not the case with the simple-toothed rodents.
We now come to the subdivisions of the Simplicidentata. The order GLIRES has always been a puzzling one to naturalists, from the immense variety of forms, with their intricate affinities, and there is not much help to be gained from extinct forms, for such as have been found are mostly referable to existing families. The classification which I have adopted is, as I said before, that elaborated by Mr. E. R. Alston, F.G.S., F.Z.S., and reported in the 'Proceedings' of the Zoological Society for 1876. I said that he had founded it on Professor Gervais' scheme, but I see that the groundwork of the system was laid down in 1839 by Mr. G. R. Waterhouse, then curator of the Zoological Society, and it was afterwards, in 1848, taken up by Professor Gervais, and subsequently added to by Professor Brandt in 1855, and Lilljeborg in 1866. About ten years later Mr. Alston, working on the data supplied by the above, and also by Milne-Edwards, Gray, Günther, Leidy, Coues, and Dr. Peters, produced a complete system of classification, which seems to be all that is to be desired.
We have already divided the rodents into two sub-orders, to which, however, Mr. Alston adds a third, viz., Hebetidentati, or Blunt-toothed Rodents, which contains only the Mesotherium, a fossil form. We have now to subdivide the two. The Double-toothed Rodents are easily disposed of in two families—Leporidæ and Lagomyidæ. The Simple-toothed Rodents are more numerous, and consist of about eighteen families arranged under three sections, which are Sciuromorpha, or Squirrel-like Rodents, Myomorpha or Rat-like Rodents, and Hystricomorpha, or Porcupine-like Rodents. It would perhaps render it clear to the reader were I to tabulate the differences chiefly noticeable in these three sections:—
SECTION I.—SCIUROMORPHA, OR SQUIRREL-LIKE RODENTS.
Molar dentition 4—4/4—4 or 5—5/4—4. In the latter case the foremost upper molar is small; the fibula is distinct, and never united, except in some cases where it is attached to the extremity of the tibia; the zygomatic arch is formed chiefly by the malar, which is not supported beneath by a continuation of the zygomatic process of the maxillary; collar-bones perfect; upper lip cleft; the muffle small and naked; tail cylindrical and hairy (except in Castoridæ). Five families.
SECTION II.—MYOMORPHA, OR RAT-LIKE RODENTS.
Molar dentition from 3—3/3—3 to 6—6/6—6, the former being the usual number; the tibia and fibula are united for at least a third of their length. The zygomatic arch is slender, and the malar process rarely extends so far forward as in the preceding section, and is generally supported below by a continuation of the maxillary zygomatic process; collar bones are perfect (except in Lophiomyidæ); upper lip and muffle as in the last; tail cylindrical, sometimes hairy, but commonly covered with scales arranged in rings. Seven families.
SECTION III.—HYSTRICOMORPHA, OR PORCUPINE-LIKE RODENTS.
With one exception (Ctenodactylus) have four molars in each upper and lower jaw; the tibia and fibula are distinct in young and old; the zygomatic arch is stout, and the malar does not advance far forward, nor is it supported by the maxillary zygomatic process; collar-bones perfect in some; the upper lip is rarely cleft; the muffle clad with fine hair; tail hairy, sub-naked or scaly.