438 LAND MAMMALS IN THE WESTERN HEMISPHERE 



teeth and skull were very profound and far-reaching, very 

 much more so than those which took place in the joreodonts. 



In the dentition we may consider separately the develop- 

 ment of the tusks and of the grinding teeth. The first step 

 in the known series, as exemplified by \Moeritherium, was the 

 enlargement of the second incisor in each jaw to form a tusk 

 which, though actually quite long, was very small when judged 

 by the proboscidean standard. The upper tusk was directed 

 vertically downward and the lower one was procumbent, point- 

 ing almost directly forward ; the third incisor and the canine 

 were small and in the lower jaw already lost. In the next known 

 stage, ^Palceo mastodon, all of the anterior teeth, except the 

 tusks, had been suppressed ; the upper tusks were longer 

 and more curved and of an oval cross-section ; they extended 

 less directly downward and more forward, while the enamel 

 was restricted to the outer side of the tusk ; the lower tuslvs 

 were more fully procumbent than in the preceding genus. The 

 third stage, that of the lower Miocene \Gomphotherium, showed 

 the upper tusks greatly elongated and directed more forward 

 than downward, while the lower tusks were but little larger 

 than before. From the middle Miocene two phyla may be 

 distinguished by the tusks alone ; in one, which was not destined 

 to long life, the lower pair increased greatly both in length and 

 in diameter, while in the other series they rapidly diminished 

 and eventually disappeared. Even in the Pleistocene, how- 

 ever, the American jMastodon had remnants of these tusks 

 in the males. In the later jniastodons, the fstegodonts, and 

 true elephants, the upper tusks, which alone remained, lost 

 the enamel bands and attained enormous proportions, differing 

 in the various genera and species in the extent and direction 

 of curvature. An aberrant mode of tusk development was 

 to be seen in the fdinotheres, in which the upper pair was sup- 

 pressed and the lower pair enlarged and so curved that the 

 points were directed backward. 



The grinding teeth underwent much more radical and 



