MARSUPIALIA 211 



toids represent a line of descent from some still earlier 

 generalized poly pro todonts and a separate stem from the 

 Australian diprotodonts. 



Sinclair has had the most complete material on which 

 to work, and with his general grouping I have come to 

 agree. This recognizes three divisions of South American 

 Marsupials, the Didelphidae, representatives of which have 

 not yet been found in the Deseado, though occurring in 

 both the earlier and later formations; the Caenolestidae 

 represented today by Caenolestes, the only survivor of 

 the South American diprotodonts; and the Borhyaenidae 

 ( = Thylacynidae of Sinclair this name having been used to 

 indicate a much nearer relationship to the Australian Thy- 

 lacynns than I feel is warranted), which includes a large 

 range of medium to large sized animals ranging from the 

 Casamayor formation throught the Santa Cruz beds. 



The locality from which these marsupials emigrated to 

 South America and the time of their arrival is not yet 

 agreed upon, and can not be settled until much more com- 

 plete material is discovered in the Casamayor formation. 

 I feel, however, that the three groups were separate when 

 they entered South America. 



Borhyaenidae 



Ameghino has grouped in this family a considerable 

 number of genera of powerful, wolf-like carnivorous 

 marsupials, characterized by a dental formula 4 3 3 \ * ** 

 heavy heads, short limbs with usually five semidigitigrade 

 toes. The genera are mostly distinguished by the relative 

 development of the protocone on the upper molars and the 



* There is a discussion as to the homologies of the premolars of marsupials 

 and placental mammals, the one proposition being that marsupials have three 

 premolars and four molars, the other that they have four premolars and three 

 molars as in placentals. The evidence is not conclusive as to either proposi- 

 tion, but in this paper I have designated these teeth along the latter line of 

 thought. 



