HISTORY OP THE PROBOSCIDEA 



429 



had thicker and more crumpled enamel plates than in either 

 of the other species. 



The fourth of the Pleistocene proboscideans of North 

 America was a member of a different and much more ancient 

 genus, ^Mastodon, which in the Old World became extinct 

 before the end of the Pliocene. The American fMastodon 

 (fM. americanus) was thus a belated survival of an ancient 

 type, seemingly out of place even in the strange Pleistocene 

 world, which had so many bizarre creatures. The distinguish- 

 ing characteristic of the genus was in the simple, low-crowned 

 and comparatively small grinding 

 teeth, which had three or four 

 prominent transverse ridges, cov- 

 ered with heavy enamel, and, 

 usually, with no cement on the 

 crowns. As these teeth were so 

 much smaller than those of the 

 elephants, as many as three on 

 each side of each jaw might be 

 in sim\iltaneous use. In this 

 species there was no vertical suc- 

 cession of teeth, but in some of 

 the Tertiary f^nastodons such 

 succession has been observed. The long tusks were directed 

 nearly straight forward and were almost parallel, with but 

 slight curvature, the convexity downward. In the males 

 there was a short single tusk or, less commonly, a pair of such 

 tusks, in the lower jaw, which were probably not visible ex- 

 ternally ; these were the vanishing remnants of an earUer 

 stage of development, when the t™astodons had a fully 

 developed pair of lower tusks, nearly as large as the superior 

 pair. 



The skull, while essentially proboscidean, was yet much 

 lower and flatter and less dome-like than in the elephants ; 

 the thickening of the cranial bones was less extreme. The 



Fig. 227. — Last lower molar of the 

 American fMastodon. 



