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Show 1880.] DENTAL CHARACTERS OF THE CANID^*. 283 number and the kinds of teeth which existed in the earliest ancestors ot the Canidae, and that the lobate mandible is similarly inherited from them, it becomes necessary to seek, for the primitive forms of the Carnivora which probably stood in the same relation to Amphicyon as Otocyon does to Canis and Vulpes, in still older formations. Nothing is at present known of the Mammals of the Cretaceous epoch; and from the older Eocene the only forms which bear upon the present question are Arctocyon, Pterodon, and Hycenodon. Of the first too little is known to warrant speculation. With respect to the two latter, M . Filhol's observations have conclusively proved that they have as little to do with the Didelphia in dentition as in other respects; and he has described an interesting form, Cynhyce-nodon, the upper dentition of which approaches that oi Pterodon, while its mandibular teeth present resemblances to those of Cynodictis. I do not suppose, however, that Pterodon (and still less Hycenodon) lies in the direct line of ancestry of the Canidae. On the contrary, they appear to constitute a peculiar branch of the stock of the Carnivora, having closer relations to the Insectivora than are possessed by modern Carnivores. In fact, in Centetes, the molar teeth of both jaws increase in size from before backwards, and the patterns of their crowns are such that those of all the Carnivora may be readily derived from them. The trihedral prism which constitutes the chief part of the first upper molar of Centetes obviously answers to the triangular elevation on the crown of the corresponding tooth of Otocyon, which terminates in the two outer and the two inner cusps ; and the main difference between the two is that the cingulum is larger and extends much nearer to the summits of the cusps in Otocyon than in Centetes. In the mandibular teeth, again, the first molar of Centetes presents exactly the same number of cusps, disposed in the same way as in that of Otocyon, the difference between the two lying merely in the different proportions of the parts. The exact correspondence in plan of these teeth is the mere interesting, since, in Centetes, it is easy to trace the progressive changes by which the simple and primitive character of the Mammalian cheek-tooth exhibited by the most anterior praemolar passes into the complex structure of the crowns of the posterior teeth. This is particularly obvious in the lower cheek-teeth, in which the crown of the most anterior praemolar is simply tricuspidate, with the anterior and the posterior cusps very small and the apex of the principal cusp simple. In the next praemolar the principal cusp appears cleft near its apex, in consequence of the development of a small secondary cusp on its inner side; the anterior cusp is higher and the posterior both higher and thicker. In the third praemolar, and in the molars, the anterior cusp is still higher; the secondary cusp is as large as that from which it is derived, so that it answers to the anterior internal cusp, while the former principal cusp takes the place of the anterior external cusp of the typical canine tooth. The posterior cusp, become very broad, and divided by a faint median depression, represents the posterior external and posterior internal |
