120 LECTURES ON EVOLUTION m 



front crescent, a crescentic front ridge passes 

 inwards and backwards, and its inner face enlarges 

 into a strong longitudinal fold or pillar. From 

 the front part of the hinder crescent, a lack ridge 

 takes a like direction, and also has its pillar. 



The deep interspaces or valleys between these 

 ridges and the outer wall are filled by bony 

 substance, which is called cement, and coats the 

 whole tooth. 



The pattern of the worn face of each grinding 

 tooth of the lower jaw is quite different. It 

 appears to be formed of two crescent- shaped 

 ridges, the convexities of which are turned out- 

 wards. The free extremity of each crescent has a 

 pillar, and there is a large double pillar where 

 the two crescents meet. The whole structure is, 

 as it were, imbedded in cement, which fills up the 

 valleys, as in the upper grinders. 



If the grinding faces of an upper and of a lower 

 molar of the same side are applied together, it 

 will be seen that the apposed ridges are nowhere 

 parallel, but that they frequently cross ; and that 

 thus, in the act of mastication, a hard surface in 

 the one is constantly applied to a soft surface in 

 the other, and vice versa. They thus constitute a 

 grinding apparatus of great efficiency, and one 

 which is repaired as fast as it wears, owing to the 

 long-continued growth of the teeth. 



Some other peculiarities of the dentition of the 

 horse must be noticed, as they bear upon what I 



