170 



EVOLUTION AND GENETICS 



and the presence of lower tusks, it was distinctly like the elephant 

 in appearance (Fig. 96). Its teeth and skull show a greater de- 

 velopment of the typical proboscidean character than any of its 

 predecessors. 



Mastodon and Its Contemporaries. During the same period 

 Mastodon arose (Fig. 97), to persist in America until its extinction 



Fig. 96. — Restoration of Trilophodon. (From Lull, after the British Museum 



Guide to Elephants.) 



in the Pleistocene, and Tetralophodon marked the beginning of 

 another line which later produced Dihelodon. Dibelodon migrated 

 to South America and became extinct at about the same time as 

 the mastodons. 



TetralopJwdon (Fig. 98) represents the extreme development of 

 the lower jaw and the four-tusked condition in the Prohoscidea, 

 although its descendant, Dihelodon, had the lower jaw shortened 

 and without tusks. The two are different from the main line of 

 descent in the more complex structure of the molars. 



Mastodon included a number of different species, and has been 

 subdivided into other genera by some paleontologists. In general 

 these animals had the form of elephants, but they were more 



