PHYLOGENY AND RELATIONSHIPS 



141 



give definitive answers to all the problems which turn on them, and anything done 

 now will inevitably be changed, perhaps very radically changed, by future discoveries, 

 but the time is ripe for the formation of new working hypotheses as to relationships, 

 and the material is adequate for the formation of such hypotheses, provided that they 

 are based not on one collection or another and not on one aspect and approach or an- 

 other, but on the whole of the material considered from as many points of view as lie 

 within the powers of the worker. 



After studying all the known mate- 

 rial, the present writer attempted an in- 

 terpretation of the affinities of the vari- 

 ous groups of Mesozoic mammals. In the 

 Catalogue of Mesozoic Mammals in the 

 British Museum the Triassic and Juras- 

 sic mammals have been treated in some 

 detail in this connection, and in formu- 

 lating the working hj^potheses there ex- 

 pressed all of the facts published here 

 were also taken into consideration. The 

 multituberculates have been further con- 

 sidered elsewhere by Granger and Simp- 

 son (1929), and the more important 



Mesozoic insectivores by Gregory and 



Simpson (Gregory and Simpson 1926A, 



Simpson 19280). It is not necessary to 



repeat all the arguments and all the 



bases for the tentative conclusions thus 



reached. The present section therefore is 



confined to a brief consideration of the 



Mesozoic marsupials and to a still more 



brief resume of present views as to the 



probable phylogeny and relationships of 



all the Mesozoic mammals. 



Fig. 57. Lower molars of Mesozoic mammals. A, 

 Triconodonta, Priacodon. B, Multituberculata, 

 Ctenacodon. C, Symmetrodonta, Tinodon. D, Pan- 

 totheria, Dryolesies. E-F, Marsupialia, Lance di- 

 delphids. Not to scale. 



THE MESOZOIC MARSUPIALS 



The earliest known marsupial is Eodelfhis from the Belly River, upper Cretace- 

 ous. Many of the Triassic and Jurassic mammals have been believed already to have 

 been definite marsupials, but in any just conception of taxonomy a mammal is not to be 

 classified as a marsupial merely because it is ancient or primitive but only because it 

 has characters which taken together characterize the Marsupialia and no other group. 

 Ree.xami nation of the Triassic and Jurassic mammals has shown beyond apparent 

 question that none of them does show such characters. No mammals earlier than the 

 Cretaceous can reasonably be referred to the Marsupialia except on purely theoretical 



