THE TEETH OF PERISSODACTYLE VNGULATA. 319 



The outer ridge or wall is in the upper molar of the horse 

 doubly bent, the concavities looking outwards. The trans- 

 verse ridges start inwards from its anterior end and from its 

 middle, and they curve backwards as they go to such an 

 extent as to include crescentic spaces (between themselves 



FIG. 135 0). 



and the outer wall). To this we must add a vertical pillar, 

 which is slightly connected with the posterior end of each 

 crescentic edge (this pillar is in Hipparion quite detached). 



The lower molars of the horse present the double crescent, 

 just like those of the rhinoceros, save that vertical pillars 

 are attached to the posterior end of each crescent, thus 

 slightly complicating the pattern of the worn surface. The 

 interspaces of the ridges and pillar are in the horse solidly 

 filled in with cementum. The extinct ancestors of the horse 

 have the molar pattern considerably simplified, but yet 

 recognisable as being built up on the same model. 



But in an elementary handbook such as this it will only 

 serve to perplex the reader to enter into a discussion of 

 the relative probabilities of the various and incompatible 

 explanations given of the homologies of the parts of the 

 ungulate molar : suffice it that such correspondences do exist, 

 and if we had before us perfect chains of molars from every 



(*) Molar tooth of a Horse, showing the pattern of its grinding surface. 



