36 THE HORSE AND ITS RELATIVES 



These are most developed in the extinct three- 

 toed hipparion, as shown in B of the same illustra- 

 tion, in which the cement filling the pits is white. 

 Further complexity is produced by vertical flutings 

 on the inner side of the crown, which result in the 

 production of the two semi-isolated pillars marked 

 p and hy in the aforesaid figures. Of these two 

 inner pillars by far the more important from a 

 systematic point of view is the front or anterior 

 one (p in the illustrations), for it affords an import- 

 ant character in the definition of the genus Equus. 

 In all the existing members of the horse family this 

 pillar is connected by a narrow isthmus with the 

 main body of the tooth ; and the fore-and-aft 

 diameter of its worn surface is considerably longer 

 than the transverse one. A pillar of this type is 

 termed a broad one ; but there are certain extinct 

 horses in which, while the pillar remains connected 

 with the body of the tooth, its two diameters are 

 nearly equal ; in teeth of this type the anterior 

 pillar is said to be narrow. 



In the three-toed hipparions (B of the illus- 

 tration on page 33) a totally different condition 

 obtains, the anterior pillar, which is of the narrow 

 type, being completely surrounded by a ring of 

 enamel, so that its central core of dentine is cut off 

 from the dentine of the main body of the tooth. 

 To put the matter shortly, it may be said that while 

 the upper molars of the horse and its immediate 



