26 



plane is nearly horizontal. From the middle of its outer margin the ridge is 

 continued vertically upwards, which hounds anteriorly the depression described 

 in the side-view of the skull. 



The tympanic cavity is not very deep nor very large : it is hounded posteriorly 

 hy the mastoid and rugged cubical process of the petro-temporal ; internally by 

 the smoother conical process of the petro-temporal and by the rough protu- 

 berance of the sphenoid ; anteriorly it is continued forwards and downwards hy 

 the Eustachian depression upon the posterior base of the pterygoid ; externally 

 it opens upon the smooth depression between the stylo-hyal and the glenoid ar- 

 ticulations of the temporal : an elliptical depression, admitting the end of the 

 little finger, leads from the roof of the tympanic cavity into the cellular structure 

 of the base of the zygoma. The tympanic bone is wanting : it has not been 

 broken off, as a mere process of the temporal ; hut must have been an inde- 

 pendent detached ossicle, and probably similar in shape to that of the Glosso- 

 therium and Orycteropus. 



The concave outer side of the base of the pterygoid, above the large foramen 

 for the third division of the fifth, is continued into the flat, horizontal arti- 

 cular surface for the lower jaw at the base of the zygoma. This surface is ob- 

 scurely bounded by a slight convex prominence of the bone behind ; it passes 

 externally into the narrower, flattened surface, which is continued along the 

 under part of the zygoma, to its termination at the articular surface for the 

 malar bone. 



The base of the skull is naturally divided into three regions, — the posterior one 

 behind the pterygoids ; the middle one including the pterygoids, and formed 

 chiefly by the wide posterior depression and aperture of the nasal cavity ; and the 

 anterior one formed hy the dental alveoh and bony palate. The anterior region is 

 of a triangular form, with the base turned forwards and hounding the floor of the 

 skull anteriorly by a convex curve, indented by a median angular notch ; the 

 sides of the triangle are shghtly concave, and converge to the posterior boundaiy 

 of the palate, the apex of the triangle being here cut off by an entering notch, 

 the sides of which receive the pterygoids ; five alveoli are situated on each side 

 of the palate, and the intermediate plate of bone is slightly convex and sculp- 

 tured- by vascular grooves and foramina. 



The posterior alveolus is situated one inch and a half from the apex of the 



