THE HEAD AND NECK 



123 



normal conditions, supplies sufficient grinding-surface 

 to last the lifetime of the animal, a neck and roots are 

 formed, and the tooth is reduced to the condition of 

 that of the brachydont ancestor (see b and c, fig. 20). 



It is perfectly clear that this lengthening of the 

 crown adds greatly to the power of the teeth as organs 

 of mastication, and enables the animals in which it has 

 taken place to find their sustenance among the com- 

 paratively dry and harsh herbage of the plains, the 

 stalks of which often contain much hard mineral 

 matter, instead of being limited to the soft and succu- 

 lent vegetable productions of the marshes and forests, 

 in which the primitive brachydont forms of Ungulates 

 mostly dwelt. 



The hypsodont, or high-crowned type of tooth, which 

 may be looked upon as an intermediate condition 

 between the rooted and the ever-growing type, is by no 

 means peculiar to the molars of the horse. It occurs, 

 as already mentioned, in the incisors of the same 

 animal. It is also met with, in various degrees, in the 

 more recently-developed forms of the rhinoceros family 

 (though not in the tapirs), and in some of the most 

 specialised of the Artiodactyles, as the ox and the sheep, 

 though not attaining in those animals to the same 

 development which it does in the horse. 



As there are some differences in the details of the 

 structure of the premolars and molars of the upper and 



