558 SE. f . AJIEGHISO OX THE [-^^'l)' 2, 



Oil the outer side of the anterior lobe of tlie same tooth there 

 can also be seen a small enamel ridge or dwjidum (tig. 2 a, c), the 

 presence of which must not be overlooked. 



Fig. 2. 



.(- 



f- 



Trotcudidclph/s pracurwr : sixth right lower iiiohir, external (n) and superior {h) 

 aspect, eight tiiues aat. size. — Lower Cretaceous ; Patagonia. 



Finding thus in the teeth of such an old animal a cooiplication 

 which is said to be the result of a successive addition of cusps 

 through geological ages, we have a right to doubt this latter asser- 

 tion, 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 o£ 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 'nanner as in Proleodidelphys, the 

 anterior lobe sho^^iug also the same ciiigulum (c). In these 

 animals, therefoi-e, the complication in question is not of recent 

 origin, but an inheritance of their oldest known predecessor. 



Proteodidelj^iliya is a representative of the family Microbiotheridae. 

 In several of rny publications I have had the opportunity of 

 showing that this family constitutes the stem not only of the 

 Didelphyidfe but equally of the Sparassodouta, Dasyurida3, Creo- 

 donta, insectivora, and Carnivora. The lower molars of these 

 different groups are merely modifications, generally not very 

 considerable, of the molars of Protc.odideJphiis. In the Eocene 

 Microbiotheridffi the modiKcations are iusigniticant. The molars 

 of Cretaceous Sparassodouta still preserve the vestiges of all the 

 cusps, which in their Eocene descendants are reduced by the 

 cHsappearance 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 Australiau Dasyurida?, cusp ai being still 

 present in Dasyvrus, whilst it has disappeared in Thyhicimis. 

 The six cusps characteristic of Didelphyida> arc known to exist in 

 most of the genera of Creodonta (I'aJa oni' tis, Froviverra, Myavis, 

 &c.), the predecessors of the Carni> ora ; they equally persist in 

 many of the latter, esjiecially in Procyonida\ recent (Proryon, 

 JS^asiia) and fossil (Cyonasua), in primitive Cauidte (Cynodon) and 

 Ursidae, in the Yiverridse, &c. In some genera of Carnivora this 

 form has scarcely undergone any appreciable moditication : on 

 examining the first inferior molar of Cyonasua (fig. 3), one is 

 struck by its perfect resemblance to the corresponding tooth of 

 ProtcodAdeliiliys and Didcljjhys. The same tooth-pattern is met 



