UPPER CRETACEOUS MARSUPIALIA 115 



occipital exposure of mastoid large. Palatal vacuities usually present. Optic foramen 

 confluent with sphenorbital fissure. Internal carotid piercing basisphcnoid (save in 

 Acrobaivs). Epipubic bones generally present. 



Distribution. — Cretaceous, North and South America. Paleocene, Europe and 

 North America. Eocene and Oligocene, Europe and North and South America. Mio- 

 cene, Europe and South America. Pliocene, South America and Australia. Pleistocene 

 and Recent, North and South America and Australia. This known distribution is 

 obviously fragmentary. They were probably continuously present in North and South 

 America and in Australia from the Cretaceous to recent time and in Europe from the 

 Cretaceous to the end of the Miocene. 



Various of the forms here gathered together under the Marsupialia have been 

 referred to all three of the major recent groups of mammals and also to the Jurassic 

 Trituberculata (Pantotheria). Marsh (especially 1892B) recognized the fact that most 

 of them are marsupials and placed them in a family Cimolestidae, but he believed that 

 insectivores and pantotheres were also present. Osborn (1893) at first considered 

 Thlaeodon as a possible monotreme and suspended judgment as regards the other gen- 

 era. Later, however, he considered certain genera now believed to be didelphid as 

 "capable of giving origin to the teeth of the Amblypoda." (Osborn 1898, p. 172.) In 

 the latest study of the problem, Matthew (1916, p. 481) states: "While for various 

 reasons it seemed probable that most or all of these Cretaceous trituberculates were 

 marsupials, there was no conclusive proof of it. On the other hand there was not and is 

 not any valid evidence for placing any of them in the placental group," and he goes on 

 to prove that the only two genera which he considers as really determinable are cer- 

 tainly marsupial. 



The necessarily confused taxonomic status of these fragmentary remains, so 

 strongly stressed by Matthew, is unfortunate, but does not alter the fact that reason- 

 ably certain conclusions as to the diversity and the broader phylogenetic and adapta- 

 tional features of the fauna are possible. The present writer believes, with Matthew, 

 that the majority of the "trituberculates" are demonstrably marsupials, but insists that 

 placentals are also present, Gypsonictofs certainly and Batodon and Telacodon prob- 

 ably belonging to the higher subclass. 



Osborn's method of approaching the revision of the Cretaceous marsupials still 

 seems the most practical. He emphasized (1893) the importance of basing taxonomic 

 results chiefly on the upper molars, so far as possible, and of not attempting to refer 

 teeth from other parts of the dentition not actually found in association. In the present 

 memoir the taxonomy is based chiefly on upper molars save where names have already 

 been given by Marsh to lower molars. Warrantable exceptions are to be made only of 

 the few relatively complete lower jaws (e.g., Eodelfhis ciilleri) which really do pro- 

 vide an adequate taxonomic basis. Teeth are referred to a definite genus only when 

 directly comparable with the genoholotype. As with the multituberculates, the genera 

 are used in a very broad sense, in some cases certainly including two or more related 

 genera, and no attempt is usually made to distinguish species beyond the type species 

 of genera. 



