254 THE STRUCTURAL EVIDENCE OF EYOLUTIOIT. 



when unworn, present four principal tubercles, wliicli are openly 

 V-shaped in section, and which are separated by open valleys. 



The latter are deeper than 

 those of the truly Btmo- 

 clont genera, but much 

 shallower than those of the 

 typical Selenodonts. In 

 Oreodon the valleys are 

 somewhat deepened and 

 the crescents elevated, while 

 in the deer the same infold- 

 ing is carried still further. 

 In the Cavicornia the type 

 reaches its fullest expres- 

 sion in the loss of the shoul- 

 der at the base of the crown, 

 the great elougation of the 

 latter, and corresponding- 

 ly deep infolding of the 

 terminal valleys. 



Professor Lartet* states 

 that the most ancient deer 

 have very short - crowned 

 molars, and the depressions 

 on the surface are so shallow that the bottom is always visible, 

 while in the CervidcB of the more recent Tertiary periods, and 



especially the Plistocene and living species, 

 these same cavities are so deep that what- 



FiG. 20. — Hippopotamus. 



Fig. 21. — Hyopotamus velaumis. 



Fig. 22. — ITyopotamus americanus. 



Fig. 23. — Procamelns robust us. 



ever be the state of attrition, the bottom can not be seen. This, 

 he says, is a perfectly reliable rule for distinguishing the ancient 

 from the more modern forms of deer, and can be applied to other 

 animals as well as the CervidaB. 



* u 



Comptes Rendus," 1868, p. 1119. 



