x.] CARNIVORA. 167 



obvious modifications from this type relate to the compara- 

 tive length and compression or width of the facial portion, 

 the strength and curve of the zygomatic arch, and the extent 

 to which the various ridges and processes for the attachment 

 of muscles are developed. Thus the Cats have short and 

 round skulls, with wide zygomatic arches ; and in the Bears 

 (especially the Polar Bear, Ursus maritimus) the whole 

 skull is elongated, and the nasal cavities are greatly enlarged 

 as compared with the brain-case, and the maxillo-turbinal 

 bones are correspondingly developed. 



But there are certain other modifications of the cranial 

 bones, which, being less obviously adaptive to functional 

 purposes, and being constantly associated with structural 

 modifications in other parts of the body, are of considerable 

 value in classifying the members of the group. Of these 

 the most important are related to the form and structure of 

 the auditory bulla, and the surrounding parts of the base of 

 the cranium. 



In the Bears, the auditory bulla is comparatively little 

 inflated. It consists of a single bone (tympanic), readily 

 detached from the cranium in skulls of young animals. Its 

 form is more or less triangular, being broad and nearly 

 straight at the inner edge, and produced outwards into a 

 considerably elongated floor of the external auditory meatus. 

 Its greatest prominence is along the inner border; from 

 this it gradually slopes away towards the meatus. The 

 entrance of the carotid canal is a considerable circular 

 foramen, near the hinder part of the inner edge of the bulla. 

 In old animals it is partly concealed by the prominent lip of 

 the basioccipital, which abuts against the inner edge of the 

 bulla ; and by the growth of this, and the paroccipital pro- 

 cess, it becomes almost included in the deep fossa leading 

 to the foramen lacerum posterius. When a section is made 



