XIII 



PHYLUM CHORDATA 



553 



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

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

 niagnum {f.vi) is no longer situated at the posterior extremity of the 

 skull as in other J\[ainmals, 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 



Fio. 1170. — Skull (if Man. Letters as in Fig. lljj. In addition, «. angle of mandible ; 

 c. fi. crista galli, a process of the mesethmoid ; sh. styloid process ; st. sella turcica. 

 (After Flower.) 



horizontcil, and the 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 beneath the posterior 

 portion of the cerebral fossa : the olfactory fossa is comparatively 

 small. (See Fig. 1126, B.) 



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 



