256 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



in Elotherium, Halitherium, and Trichechus tliey are few ; in 

 Trachytherium and Dinotlierium more numerous ; in Mastodon, 



Stegodon, and Elephas, most 

 numerous. The tubercles are 

 united into serrated cross- 

 erests in HalWierium, the ex- 

 tinct sea-cow ; in Trachythe- 

 rium, another fossil ally of the 

 Manati, the tubercles are not 

 united. The succession from 

 Mastodon to Elephas may be 

 represented by the accompany- 

 ,, , ., inff fiffures : Fio^. 26 (from 



iiG. 26. — Mastodon angushdens. . . . , « ,V 



Cuvier) is a molar of M. an- 

 gustidens, where, beside the principal tubercles, numerous lesser 

 ones appear. Fig. 27 represents Mastodon ohioticus, in which 



Fig. 27. — Mastodon ohioticus. 



Fig. 28. — Dinotherium giganteum. 



the opposite tubercles are nearly united into transverse crests. 

 In Di7iotherium (Fig. 28) and Elephas (Fig. 29) the union is 

 complete. The relation of these 

 genera has been described as 

 one of " inexact parallelism " ; a 

 condition supposed by the writer 

 to depend on modification in 

 descent under the law of acceler- 

 ation. The language used is : * 

 '* The young tooth of Elephas, 

 moreover, is represented by a 

 series of independent parallel 



Fig. 29. — Elephas indicus. 



lamina? at first, which, when they unite, form a series of crests 



* "Origin of Genera," p. 17. 



