MASTODON. 33 



America,, continued in a comparatively primitive condition, is pier- 

 to be found in their considerable isolation and freedom from *Q a ^S 

 competition. One South American species, M. andium, 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 

 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, and the presence in 

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

 and E. primiyenius) is due to immigration from Asia. No 

 species of Elephas reached South America. 



To return to the series of stages of development found in Pier- 

 Southern Asia, the first species that need be considered is ^^le 

 Mastodon cautleyi, which in the character of its teeth is nearly cases 

 related to Tetrabelodon longirostris, but the ridges of the molars 23 > 24 * 

 are comparatively higher. There is, however, as yet no cement 

 in the valleys (fig. 19), which are more or less obstructed by 

 small tubercles, and the inner cusps wear into a trefoil pattern, 



Fig. 20. 



Grinding surface of a lower molar tooth of Mastodon sivulensis. 

 From the Lower Pliocene of the Siwalik Hills, India, f nat. size. 



as in Tetrabelodon angustidens and T. lonyirostris. While, 

 however, the anterior molars are almost identical with those of 

 T. longirostris, the posterior lower molar is very similar to that 



