xiii PHYLUM CHORDATA 537 



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

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

 skull 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 

 horizontal, 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. 1138, D.) 



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

 minent 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 represented 

 by slight eminences the jugular eminences. There is no auditory 

 bulla ; the mastoid portion of the periotic projects downwards as 

 a prominent mastoid process. The periotic, tympanic, and squa- 

 mosal early fuse into one bone the temporal bone. The post-glenoid 

 process is very slightly developed. The whole facial region is 

 relatively small. The orbits, which are of moderate size, are 

 directed forwards ; the bony margin is complete, and a plate of 

 bone, developed partly from the jugal, partly from the alisphenoid, 

 almost completely cuts it off from the temporal fossa, leaving only 

 a small aperture of communication the spheno-maxillary fissure. 

 The frontal suture usually early disappears. The nasals are 

 rarely fused. The suture between the premaxillse and the maxillae 

 becomes obliterated at an early stage, so that the entire upper jaw 

 appears to consist of a single bone. A peculiar spine, the nasal 

 spine, is developed in the middle line below the nasal opening. The 

 most marked feature of the mandible is the presence of a prominence, 

 the mental prominence, in the lower part of the symphysial region 

 ($.). The stylo-hyal nearly always becomes fused together with 

 the tympano-hyal to the periotic and tympanic, giving rise to a 

 slender process the styloid process (sn.) projecting downwards 

 from the base of the skull. 



None of the other Primates have a cranial capacity approaching 

 that of Man ; and those modifications in the shape of the skull 

 which are the concomitants of the great development of the brain 

 in the human species are accordingly not recognisable, or are much 

 less strongly marked. The various fossae of the cranium, as a 

 rule, occupy, however, the same relative positions as in Man ; the 

 cerebellar fossa is entirely beneath the cerebral ; and the ethmoidal 

 plane, and that of the foramen magnum (occipital plane), are usually 

 both horizontal or nearly so. In all the Simiidse, with the exception 

 of the Orang, the frontals meet in the middle line below, over 



