THE HEAD AND NECK. 129 



of all teeth in the early forms were very low, or 

 short from above downwards, and therefore bnt 

 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 develop- 

 ing, as soon as the neck once rose fairly above the 

 margin of the bone, the tooth remained permanently 

 in this position. The term " brachydont n expresses 

 this condition of tooth, which was universal in Peris- 

 sod actyles up to and including the Anchitherium of 

 the Miocene epoch (see a, 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 being formed 

 of enamel. When the ridges became worn down by 

 the friction of hard particles of food interposed be- 

 tween the opposing teeth, the dentine of the interior 

 was exposed, forming islands surrounded by enamel. 

 As the wear continued and reached the bottom of 

 the valleys, all the enamel coating disappeared from 

 the upper surface, and nothing remained but a plane 

 surf ace of comparatively soft dentine, surrounded at 

 the circumference by the enamel. With the progress 

 of time, however, individual succeeded individual, in 

 each of which, probably by insensible degrees, the 

 crowns of the teeth became longer, the valleys deeper, 



