POSITION AND STRUCTURE OF HORSE 37 



relatives have the anterior pillar of a peninsular 

 type, in the three-toed hipparions it assumes a com- 

 pletely insular form. 



The lower cheek-teeth of the horse have much 

 narrower crowns than the upper ones ; but the 

 foldings on their crowns are of the same general 

 type, although in a reversed way, the portion 

 corresponding to the inner pillars being on the 

 outer side of the tooth. It will not be necessary 

 to describe these teeth in detail. 



In a general way the three upper molars of 

 the horse correspond in structure with those of the 

 ox. In each, as is best seen when the teeth are 

 in an unworn condition, there is a pair of central 

 pits or islands on the crown, around which are 

 four sub-crescentic columns ; but whereas the pits 

 are almost completely filled with cement in the 

 horse, in the ox they remain more or less open. 

 Such a tooth may be described as consisting of two 

 lateral lobes, each with a single central pit. 



In the horse, as shown in the figures in 

 plate v., the upper premolars are of the same 

 structure as the molars ; the last premolar being 

 in some cases even larger than the first molar. 

 In the ox, on the other hand, the upper premolars 

 are smaller in size and simpler in structure than 

 the molars ; each of the former consisting of only 

 a single lobe, with one central pit, although this 

 lobe is somewhat larger than one of those of the 



