130 THE STEUCTUEE OF THE HOESE. 



and the ridges not only more elevated, but more 

 curved and complex in arrangement. To give sup- 

 port to these high ridges, and to save them from 

 breaking in use, the valleys and cavities between 

 them, became filled up to the top with cement, wilich 

 was also packed round the whole outer surface of 

 the enameled crown, and as the tooth wore down the 

 result was an admirable grinding-surface, consisting 

 of patches and islands of the two softer substances 

 dentine and cement separated by variously re- 

 duplicated and contorted lines of intensely hard en- 

 amel, the greater resistance of which to the attrition 

 of the food caused it to project slightly above the 

 rest of the surface (see section of the tooth of Hip- 

 parion, Fig. 10, p. 71). To this lengthened form of 

 crown the term " hypsodont " is applied. Instead of 

 contracting into a neck and forming roots, its sides 

 continue parallel for a considerable depth in the 

 socket, and as the surface wears away the whole tooth 

 slowly pushes up, and maintains the grinding-edge 

 constantly at the same level above the alveolar bor- 

 der, much as in the perpetually-growing front teeth 

 of many rodents, which never contract at the base to 

 form roots, but continue throughout the lif e of the 

 animal to grow from below to the same extent as 

 they are worn away at the outer, or cutting-edge. 

 But the horses have not quite attained this condition. 



