HISTORY OF THE MARSUPIALIA 



643 



The suborder was thus preeminently a Mesozoic one and, 



with the doubtful exception of South America, it is not known 



to have passed beyond the limits of the Paleocene. There 



is not the least likelihood 



that any existing mammals 



were derived from the f Allo- 



theria. 



While the jAllotheria 

 have an antiquity at least 

 equal to that of any other 

 mammals known, there were 

 other groups in the Jurassic 

 and Cretaceous, which, so 

 far as may be judged from 

 teeth alone, would seem to 

 have been ancestral to the 



other marsupials and to the placentals. It would serve no 

 useful purpose to describe these minute creatures, which are 

 so very incompletely known, though to the specialist they are 

 of the highest interest. The genera found in the Triassic 

 of North Carolina may or may not represent the primitive 

 mammalian stock. 



Fig. 304. — Head of ^Plilodus grticili.s, about 

 natural size. Restored from a skull in the 

 United States National Museum. 



The question of the origin of the Mammalia is still involved 

 in great obscurity, and the most divergent opinions are held 

 concerning it. It remains an unsolved problem whether the 

 mammals were all descended from a common stock, or have 

 been derived from two independent lines of ancestry, or, in 

 technical phrase, whether the class is monophyletic or diphy- 

 letic. Assuming, as seems most probable from present know- 

 ledge, that the mammals are monophyletic, the question next 

 arises : From what lower vertebrates are they descended ? 

 A great controversial literature has grown up around this 

 problem, one party regarding the Amphibia and the other the 

 Reptiha as the parent group. The palaeontological evidence, 



