HISTORY OF THE MARSUPIALIA 



WS 



The suborder was thus preeminenth' a ^lesozoie one and, 



with the doubtful exception of South .Imerica, it is not known 



to have passed beyond the hmits of the Paleocene. There 



is not the least likelihood 



that any existing mammals 



were deri^'ed from the jAllo- 



theria. 



"\^"hile the jAUotheria 

 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 Xorth Carolina may or may not represent the primitive 

 mammaUan stock. 



Fig. 304. — Head of TPtilodus graciU.^. about 

 natural size. Restored from a skull in the 

 United States National Museum. 



The question of the origin of the ^lanmialia is still involved 

 in great obscurit}', and the most divergent opinions are held 

 concerning it. It remains an unsolved problem whether the 

 mammals were aU 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 monophAdetic, the question next 

 arises : From what lower vertebrates are they descended ? 

 A great controversial literature has gro^^■n up around this 

 problem, one party regarcUng the Amphibia and the other the 

 ReptQia as the parent group. The palseontological evidence, 



