IV.] 



PROBOSCIDEANS. 



173 



simple molar teeth, in which the crowns are surmounted by low 

 transverse ridges, frequently separated into more or less distinct 

 tubercles, and with the intervening valleys open, such ridges 

 varying from three to five in number in the majority of the teeth, 



FlG. 42. LAST UPPER MOLAR TOOTH OF A MASTODON. (^ nat. size.) 



although more numerous in the last of the series. Both in Europe 

 and North America mastodons make their appearance in the 

 Miocene, although in the latter area they are unknown before the 

 Deep River beds forming the upper portion of that stage; and 

 whereas they disappeared in the Old World with the Pliocene, 

 they persisted in the New till the succeeding Plistocene age. 

 During the Pliocene, as we have seen in the last chapter, they 

 obtained an entrance into South America, so that they cannot be 

 regarded as absolutely characteristic of Arctogaea. On the whole, 

 it is probable that the group originated in the Old World, although 

 we are still in the dark as to its relationship to other ungulates. 



The only other Arctogaeic genus of proboscideans is Dino- 

 therium, which constitutes a family by itself, and is known from the 

 Miocene and Pliocene of Europe and India, but is unrepresented 

 in America. In these animals only one of the true molars has 

 three ridges, the others having but two; and tusks were present in 

 the lower jaw alone. 



A fourth subordinal group of the ungulates, which is more 

 primitive than any of the foregoing, and has been designated by 

 Professor Cope the Amblypoda, or short-footed group, has one 



