PHYLUM CHORDATA 



55;^ 



of the cavity greatly exceeds that of the basi-cranial axis. A re- 

 sult of the posterior bulging of the braiu-case is that the foramen 

 magnum (/.m) is no longer situated at tlie posterior extremity of the 

 skidl as in other Mammals, but assumes a position further forwards 

 towards the middle of the base. The anterior expansion, causing 

 a strong arching forwards of the frontal region, brings about an 

 alteration in the position of the ethmoidal plane, which, instead of 

 being perpendicular or inclined to the basi-cranial axis, becomes 



Fir;, 1170. — Skull (if Man. Letters a.s in Fig. U.j.l In ;i<Mitiiin, <'. angle of iiiAndil)]e ; 

 c. q. erista g.illi, a proeess of the mesethraoid ; sit. .styloid iivoce.ss ; s(. sella tuicica. 

 (After Flower.) 



horizontal, and tlie cribriform plate forms the middle part of the 

 floor of the anterior extension of the cranial cavity. The fossa for 

 lodgment of the cerebellum lies entirely beneatli the posterior 

 portion of the cerebral fossa : the olfactory fossa is comparatively 

 small (See Fig. 112G, D.) 



The outer surface is smooth and rounded, devoid of any prom- 

 inent ridges or crests. The occipital crest of lower Mammals 

 is represented merely by a rough, raised line — the superior curved 

 line of the occiput. The paroccipital processes are only 



