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Show 72 MR. E. R. ALSTON ON THE ORDER GLIRES. [Jan. 18, nostrils pointed above, sigmoid or linear. The ears are very generally emarginate behind; and the tail, when present, is cylindrical, hairy, scaly, or subnaked. In the division of the hystricine Rodents into families much diversity of opinion has prevailed. Mr. Waterhouse, laying too great stress on dental characters, entirely separated the Dasyproctina from the Caviina, and placed them between the Hystricina proper and the Echimyina*. Prof. Brandt reunited the two former in his family Hemionychoides, equivalent to the Subungulata of Illigerf. But the group thus formed is so ill defined that Prof. Lilljeborg found it impossible to separate it from the Hystricida%. It seems to m e that, although Mr. Waterhouse was certainly misled in entirely separating the Agoutis and Pacas from the Cavies and Capybara, they must still be ranked as distinct but allied families, and that the same value must be given to the curious form named Dinomys by Dr. Peters§. Accordingly I would recognize six families of the Hystricomorpha. Of these the first, the Octodontida, consists of three subfamilies ,- for here, I think, must be placed the Ctenodactylina, formerly associated with the Jerboas, but of which the hystricine affinities have been established by Dr. Peters||. The other subfamilies are the Octo-dontina and Echinomyina of Mr. Waterhouse. Some of the genera of the latter make a close approach to the next family, the Hystri-cida, which in its turn is composed of two very distinct subfamilies, Sphingurina and Hystricina ; for I cannot follow Professor Lilljeborg in relegating the former to the Octodontida^, principally on account of their better-developed clavicles, which are probably an adaptive peculiarity connected with their arboreal habits. Of the remaining families the Chinchillida form a small but very natural group, connected in some characters with tbe Dinomyida; and the latter, again, has close affinities with the nearly allied Dasyproctida and Caviida. These latter families in many points, as in the mode in which their incisors wear down, their emarginated palates, and the large size of their basicranial foramina, show a striking approach to the next great group of Rodents. The second suborder, G L I R E S DUPLICIDENTATI, containing only two families, is clearly less specialized than the first, and appears to be a survivor, representing a comparatively early stage in the development of the Rodent type. At birth, the upper jaw contains the normal number of incisors; but only the two inner pairs are retained; and of these the second remain very small, and are placed directly behind the large middle pair. In the mandible there is never more than one pair. Another important proof of the inferior degree of specialization in the Glires duplicidentati is the fact that the enamel of the incisors may be traced round to their posterior surface, though it is here much thinner than in front**. Of cranial characters, it may be * Nat. Hist. Mamm. ii. p. 360. t Prod. Syst. Mamm. p. 92. % Op. cit. p. 54. § Festschrift. Gesellsch. nat. Freunde, pp. 227-234 (1873). || Tr. Z.S. vii. pp. 397-409. f Op. cit. p. 51. *** Cf. Owen, Comp. Anat. Vert. iii. p. 296. |
