350 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



lower, and downwards and backwards for the upper. 

 The mechanics of this change of direction from verti- 

 cal in the primitive forms (Sciuridae) to oblique in the 



Fig. 108 Castoroides ohioensis Foster ; two-thirds natural size ; skull from 

 below, a, incisine foramen; b, pterygoid fossa ; c, internal pterygoid plates; 

 ^/.fossa in basioccipital bone; e, external auditory meatus ;/, mastoid pro- 

 cess; g, occipital condyles ; h, tympania bulla, after Hall and Wyman. 



genera with prismatic molars, is simple enough. The 

 inferior crowns when closely appressed to the supe- 

 rior, and drawn posteriorly in the direction of the long 

 axis of the jaw, press and strain the teeth in the two 



