120 



THE STEUCTUEE OE THE HOESE 



It is characteristic of a primitive condition of den- 

 tition that premolars and molars should differ in form. 

 Such a condition is, doubtless, best adapted for an 

 omnivorous or generalised mode of feeding ; but it is a 

 specialty on the part of the Perissodactyles, which was 

 acquired very early in their history, and is, no doubt, 

 in accordance with their strictly vegetarian life, that the 

 premolars have taken on the form of the true molars, 

 and have become as completely adapted as the latter for 

 the grinding function. 



Another and still more important deviation which 

 has taken place in the general condition of the molar 

 teeth from their primitive state is this. The crowns of 

 all teeth in the early forms were very low, or short from 

 above downwards, and therefore but slightly elevated 

 above the surface of the jaw. There was a distinct 

 constriction — the neck — between the crown and the root, 

 and when the tooth was developing, as soon as the neck 

 once rose fairly above the margin of the bone, the tooth 

 remained permanently in this position. The term 

 'brachydont' expresses this condition of tooth, which 

 was universal in Perissodactyles up to and including 

 the Anchitherium of the Miocene epoch (see <x, fig. 20). 

 The free surface of the crown presented cusps and 

 ridges upon it, with valleys between; but the valleys 

 were shallow, and had no deposit of cement filling 

 them, the whole exposed surface of the unworn tooth 



