144 AMERICAN MESOZOIC MAMMALIA 



ful than that of the Multituberculata. So far as now known, they became extinct at the 

 end of the Jurassic. 



The symmetrodonts are very imperfectly known. It seems a reasonable view at 

 present that they were a sterile ofishoot from near the base of the pantotherian line. 



In the upper Cretaceous, when next a real glimpse of mammalian life is afforded, 

 nothing remains of the triconodonts or symmetrodonts. The multituberculates survive 

 in changed form but are nearing the end of their history. The two great groups of 

 Tertiary and recent mammals, the Marsupialia and Placentalia, appear for the first 

 time and are probably both derived from the general pantotherian stock of the Jurassic. 

 The post-Mesozoic history of the Mammalia is almost wholly concerned with the later 

 development of these two groups. 



These views are tentatively summed up in Fig. 58. 



