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 — Castordides ohioensisYoiXex; two-thirds natural size; skull from 

 below, a, incisine foramen; /', pterygoid fossa ; r, internal pterygoid plates; 

 a', fossa in basioccipital bone; c, external auditory meatus ;/", mastoid pro- 

 cess; g, occipital condyles ; It, 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 



