132 THE STRUCTURE OF THE HORSE. 



the comparatively dry and harsli herbage of the 

 plains, the stalks of which often contain much hard 

 mineral matter, instead of being limited to the soft 

 arid succulent 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 condi- 

 tion 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 specialized 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 lower jaws, it will be necessary to describe them 

 separately. 



Of the six principal teeth which constitute the 

 upper molar series, the four middle ones (the last 

 two premolars, pm z and pm\ Fig. 16 ; and the first 

 two molars, m 1 and m 2 ) are almost exactly alike in 

 size, form, and structure, being, roughly speaking, 



