658 



ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1908. 



mastodon. The proboscis, still supported from beneath by the rigid 

 lower jaws, could only be raised and moved from side to side. The 

 neck is now quite short, so much so that were it not for the proboscis 

 and tusks this creature could not reach the ground. Both upper and 

 lower tusks show signs of wear which could only be caused by dig- 

 ging, those on one side being often much more worn than on the other. 

 The teeth have increased in size to such an extent that but two 

 adult grinders at a time can be contained in each half of the jaws. 



Tetrahdodon was a widely spread, mi- 

 gratory form, for we find species refer- 

 able to this genus not only in Europe but 

 in Africa, Asia, and in North America. 

 In Eurasia it gave rise to Mammut 

 through the loss of the lower tusks and 

 the enamel band, while in America there 

 arose Dihelodon^ which retained the 

 enamel band and which was the first proboscidian to reach South 

 America after the formation of the Central American land connec- 

 tion either late in the Miocene or in the early Pliocene. 



Fig. 15. — Tooth of Tetrabelodon 

 an<jiistidens ( X I). 



Dibelodon. 



The genus Dibelodon is known principally from the jaws, teeth, 

 and tusks, though two splendid skulls of D. andium are preserved in 

 the Museo Nacional in Buenos Aires. The upper tusks are well 

 developed, displaying an elongated spiral form, with a well-devel- 



FiG. IG. — Skull of Dibelodon andium. 



oped enamel band, but the lower jaw is quite short, though the 

 symphysis is longer and more trough-like than in the genera Mam- 

 mut and Elephas. The lower tusks have entirely disappeared, and 

 with the shortening of the jaw the trunk must have become pendant, 

 as in the modern elephants. 



The genus Dibelodon contains several species, among which are 

 Dibelodon humboldii (Cuvier), D. mirificium (Leidy), D. prcecursor 

 (Cope), and D. andium (Cuvier). Of these Dibelodon humboldii 

 and D. andium ranged into South America and were, in fact, almost 



