The treatment of these forms has been as varied as their sizes. Ameghino, with his idea that the Casamayor and Deseado beds were Cretaceous in age, groups the larger forms as a suborder, _Sparassodonta_, and considers them ancestral to the _Creodonta_; while the small forms make up his _Sarcobora_ which he considered ancestral on one side to the rodents, on the other to the diprotodont marsupials. Sinclair, after showing the marked similarity of the _Sparassodonta_ to the polyprotodont marsupials, especially the genus _Thylacynus_, abandons that term and puts them in the family _Thylacynidae_ along with the Australian forms; the _Microbiotheridae_ he finds similar to opossums and puts in the family _Didelphidae_; while the remaining small diprotodont forms he associates with _Caenolestes_, and using Ameghino’s families as subfamilies makes three divisions of the family, _Palaeothentinae_, _Garzoninae_, and _Abderitinae_. Matthew finds the sparassodonts to be true marsupials, and without phylogenetic relationship with the creodonts. Gregory diagrams the sparassodonts as coming from generalized didelphids and derives them from the same line as the Australian polyprotodonts; while the small caenolestoids 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 _Thylacynus_ than I feel is warranted), which includes a large range of medium to large sized animals ranging from the Casamayor formation throughout 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 complete 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 1 4 3 ----------,[24] 3 1 4 3