200 Bulletin American Museum of Natural History. [Vol. XXVII, 



of Marsupials show that certain Marsupials possess a true allantoic placenta 

 in addition to the larger yolk sack placenta, a fact tending to bring the 

 Marsupials much nearer to the Placentals. 



1895. Oldfield Thomas describes Ccenolestes obscurus as a "still existing 

 survivor of the Epanorthidse of Ameghino, and the representative of a new 

 familv of recent INIarsupials," and recognizes the identity of the new material 

 with the " Ili/racodon" of Tomes (preoccupied by Hijracodon Leidy). 



1899. Dollo demonstrates the adaptive radiation in the feet and argues 

 that the ancestors of the Marsupials were arboreal animals. 



1903. Bensley reviews the Australian Marsupials with reference to the 

 adaptive radiation of both feet and teeth, fully supporting Dollo's hypothesis 

 and showing further that the Didelphidne are structurally prototypal to all 

 the remaining families. 



1903. Ameghino figures the lower jaws and dentition of a number of 

 fossil Patagonian Diprotodonts. Of these several genera (Propoli/mastodon 

 Polijdolops, etc.), resemble the ]Multituberculate family Polymastodontidae 

 in certain characters (p. 212). 



1905. Sinclair (Mem. Princeton Univ. Exped. to Patagonia), mono- 

 graphs the Marsupialia of the Santa Cruz Beds, using the material collected 

 by Hatcher. He demonstrates that the carnivorous forms were related to the 

 Tasmanian Wolf and fall within the family Thylacynidpe, that the Diproto- 

 dont families Epanorthidfe, Garzonidfe, etc. of Ameghino were all closely 

 related to the existing Ccenolestes and that Microbiotherium was closely allied 

 to the smaller species of South American opossums. 



1909. Gidley describes the skull of the INIultituberculate genus Ptilodus, 

 demonstrates the characters of the upper dentition in the Plagiaulacidse, 

 and shows that the ]Multituberculates are structurally Diprotodont Marsu- 

 pials. 



n. The Polyprotodontia. 



The Theories of DoUo and Bensley. 



That the ancestors of the INIarsupials possessed a grasping type of pes 

 and were therefore fitted for arboreal habits was the conclusion of Huxley 

 (1880) and Dollo (1899). Bensley (1901, 1903) developed this idea by apply- 

 ing to the prol)lem of the Marsupials those principles of dental evolution 

 which had already been worked out in the study of Tertiary Placentals. 

 Among Bensley's chief conclusions were the following: 



(1) The modern Polyprotodonts and Diprotodonts are closely related 

 in structure and were probably derived from a single type. 



