322 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



cause the canine in the ungulate series diminishes in 

 size, and does not, therefore, draw the inferior molars 

 forwards by wedging on the superior molar, as in the 

 Carnivora, but allows free scope to the posterior trac- 

 tion of the temporal muscle in its exercise on the lower 

 jaw. 



In those forms which masticate from the inside out- 

 wards (the ental type), the cusps of the inferior molars, 

 passing between those of the superior molars, would 

 tend to flatten the sides on which they exerted fric- 

 tion, and to extend those sides outwards beyond the 

 median apex of the cusp. (Fig. 90.) The result would 

 be, and taking into view the yielding of the tissue to 



Fig. 90. — Cusps of superior premolars and molars : a, external cusps of 

 molar of Sarcothraustes ; b, of Phenacodus ; c, of Anthracotherium ; </, of 

 Oreodon ; c, half of inferior molar of Cervus;/", superior premolar of Cory- 

 phodon ; from Ryder. 



such strain, has been, to modify the shape of the cusp 

 by pushing its side walls, so that a horizontal section 

 of it would become successively more and more cres- 

 centic. The effect on the inferior teeth would be to 

 produce the same result in their external cusps, but in 

 the opposite direction. The sides of the cusps would 

 be pushed inwards, past the apex, giving a crescentic 

 section more or less perfect, as the operation of the 

 cause had been of long or short duration. The result 

 of the lateral movement in mastication may be under- 

 stood by reference to the accompanying cut, Fig. 91. 

 The external crescents of the inferior molars (^) are 

 seen to pass between the internal crescents of the 



