558 SB. i\ AMEGHINO ON THE [MilJ 2, 



On the outer side of the anterior lobe of the same tooth there 

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

 presence of which must not be overlooked. 



Kg. 2. 



clQ. 



Protcvdiddphys pracursor : sixth right lower molar, external (a) and superior {b) 

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



Finding thus in the 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 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 of Mammalia. 



Let us begin with recent Didelphyidse, the unworn molars of 

 which are not only sextuberculate, but also exhibit these tubercles 

 (cusps) disposed in the same inanner as in Proteodidelphys, the 

 anterior lobe showing also the same ciiigulum (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 Microbiotheridse. 

 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 Proteodidel])liys. In the 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 Dasyuridse, cusp ai being still 

 present in Dasyurus, whilst it has disappeared in Thylacinus. 

 The six cusps characteristic of Didelphyidte are known to exist in 

 most of the genera of Creodonta (Falo'onutis, Proviverra, Myacis, 

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

 many of the latter, especially in Procyonida% recent (Procyon, 

 Nasva) and fossil (Cyonasua), in primitive Canidse (Cynodon) and 

 TJrsidse, in the Viverridse, &c. In some genera of Carnivora this 

 form has scarcely undergone any appreciable modification: on 

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

 struck by its perfect resemblance to the corresponding tooth of 

 Proteodidelphys and Didclphys. The same tooth-pattern is met 



