34 ELEPHANTS. 



symphysis greatly elongated (e. g., T. giganteus). These with the 

 later Mastodons probably represent several waves of immigration 

 from Asia. Somewhere about the beginning of the Pliocene 

 some of these North American forms, probably short-jawed 

 Tetrabelodonts, migrated into South America, where several 

 peculiar species are found which persisted with little change till 

 the Pleistocene. Probably the reason why these Mastodons, 

 as well as M. americanus (Stand B) of North America, con- 

 tinued in a comparatively primitive condition is to be found 

 in their isolation and freedom from competition. One Soutli 

 American species, M. andium (Pier-cases 39, 40), is par- 

 ticularly interesting on account of its variability, especially in 

 the length, of the chin and in the presence or absence of the 

 lower tusks. Some of these differences are due to sex and,. 



Fig. 20. 



Vertical longitudinal section of a molar tooth of a Mastodon, showing the 

 low crown, the open valleys between the cross-ridg'es, and the thick 

 enamel (I), c., dentine. | nat. size. 



perhaps age, but the variations are no doubt mainly due 

 to the fact that the symphysis and lower tusks were undergoing 

 reduction and that the latter were about to disappear altogether. 

 Neither in North nor in South America does it appear that the 

 Mastodons gave rise to more advanced types, arid the presence in 

 the Pleistocene of North America of true elephants (E. columbi 

 and E. primigenius) is due to immigration from Asia. No- 

 species of Elephas reached South America. 



To return to the scries of stages of development found in 

 Southern Asia, the first species that need be considered is 

 Mastodon cautleyi (Table-case 23), which in the character of its 



