7 '/it- Rise of the Mammals 



stopped, cut off iu its prime, and the titano- 

 theres ceased to be. While this stopping seems 

 abrupt it was no doubt gradual, extending over 

 long years, during which the great brutes be- 

 came more and more rare until there were 

 none. Probably this was due to over-speciali- 

 zation in size, while their teeth failed to change 

 with the changing vegetation ; for the teeth 

 of these beasts were, like themselves, simple, 

 with enamel merely spread over the surface in- 

 stead of forming deep plates in the soft dentine, 

 so that the face of the tooth was kept rough by 

 use and was literally a grinder. Thus the teeth 

 of the titanotheres were, from a mechanical 

 standpoint, poor, and adapted only to coarse, 

 succulent vegetation, for the coarse molars 

 crushed rather than ground; and when the 

 enamel was worn from the face of the tooth 

 it wasted away rapidly save at the edges. At 

 the same time there were no cutting teeth 

 in the front of the jaw, only two or four use- 

 less little round incisors, and whatever crop- 

 ping was done must have been with the lips 

 entirely. 



239 



