The Rise of the Mammals 



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

 theres ceased to be. AVhile 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 ^vorn 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 fi^ont of the jaw, only two or four use- 

 less little round incisors, and whatever crop- 

 ping was done must have been with the li]38 

 entirely. 



239 



