| OCR Text |
Show 558 SB. F. AMEGHINO ON THE [May 2, On the outer side of the anterior lobe of the same tooth can also be seen a small enamel ridge or cingulum (fig. 2 a, c), the presence of which must not be overlooked. Fig. 2. a h T Proteodidelphys pracursor : sixth right lower molar, external (a) and superior (b) aspect, eight times nat. size.-Lower Cretaceous ; Patagonia. Finding thus in tbe teeth of such an old animal a complication which is said to be the result of a successive addition of cusps through geological ages, we have a right to doubt this latter assertion, and to assume as more probable that we are in presence of a primitive conformation, the vestiges of which are to be traced in nearly all the orders of Mammalia. Let us begin with recent Didelphyidae, the unworn molars of which are not only sextuberculate, but also exhibit these tubercles (cusps) disposed in the same manner as in Proteodidelphys, the anterior lobe showing also the same cingulum (c). In these animals, therefore, the complication in question is not of recent origin, but an inheritance of their oldest known predecessor. Proteodidelphys is a representative of the family Microbiotheridae. In several of ray publications I have had the opportunity of showing that this family constitutes the stem not only of the Didelphyidae but equally of the Sparassodonta, Dasyuridae, Creo-donta, Insectivora, and Carnivora. The lower molars of these different groups are merely modifications, generally not very considerable, of the molars of Proteodidelphys. In tbe Eocene Microbiotheridae the modifications are insignificant. The molars of Cretaceous Sparassodonta still preserve the vestiges of all the cusps, which in their Eocene descendants are reduced by the disappearance of cusp ai, or its fusion with ae, followed by the atrophy of the posterior lobe and its corresponding cusps. * The same is to be seen in the Australian Dasyuridae, cusp ai being still present in Dasyurus, whilst it has disappeared in Thi/lacinus. The six cusps characteristic of Didelpbyidse arc known to exist in most of the genera of Creodonta (Palceonvtis, Proviverra, Myacis, &c), the predecessors of the Carnh ora; they equally persist in many of the latter, especially in Procyonidae, recent (Procyon, Nasva) and fossil (Cyonasua), in primitive Canidae (Cgnodon) and Ursidae, in the Viverridae, &c. In some genera of Carnivora this form has scarcely undergone any appreciable modification : on examining the first inferior molar of Cyonasua (fig. 3), one is struck by its perfect resemblance to the corresponding tooth of Proteodidelphys and Didelphys. The same tooth-pattern is met |
