MARSUPIALIA 211 
toids represent a line of descent from some still earlier 
generalized polyprotodonts 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- 
lacynus 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 **77",* 
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. 
