268 ON THE STRUCTURE OF THE SKULL. 



posterior diameter of which bones is in the middle line, as they 

 are not separated, posteriorly, by the supra-occipital, or, ante- 

 riorly, by the frontals. 



The orbito-sphenoids are large, and enter into the com- 

 position of the front wall of the skull. The alisphenoids are 

 also large, and contribute to the formation of the side walls, as 

 well as of the base, of the skull. 



The squamosal appears in the interior of the cranium between 

 the parietal, supra-occipital, and periotic, with which last it is 

 not anchylosed. 



The periotic, a large and dense ossification, has a very pecu- 

 liar form, being divided into an inner portion, corresponding 

 with the pars petrosa, and an outer thick mass which answers to 

 the teg men tympani and pars mastoidea. 



The tympanic is a mere ring of bone, open above, and having 

 a thicker anterior than posterior cms. It is by the former that 

 it is more especially attached to the periotic, though the hinder 

 thinner eras also becomes anchylosed with that bone. 



The squamosal unites behind and below with the ex-occipital, 

 but leaves a space, superiorly, in which the pars mastoidea 

 appears on the exterior of the skull. The malar process of the 

 squamosal is exceedingly thick, and extends far forwards as well 

 as transversely outwards. The frontals send very large processes 

 downwards and forwards, as in the Elephant and Tapir, which 

 are not only met by the maxilla?, as in the latter animal, but 

 also meet, and indeed are covered by, the nasal processes of the 

 preruaxilla?. The lachrymals are large, but imperforate. The 

 jugals, thick and curved, are connected with them. 



The very small nasal bones are fixed by the greater part of 

 their under surfaces to the anterior half of the ethmoid, beyond 

 which they project but little, so that almost the whole of the 

 vast anterior nasal aperture is, in the skeleton, uncovered. 

 The premaxillse are enormous, and constitute a large propor- 

 tion of the lateral margins of the upper jaw as well as the 

 whole of its anterior region. Their ascending, or nasal, processes 

 are produced forwards instead of downwards, so that the point 

 which corresponds with the spina nasalis anterior in Man is 

 nearly on a level with the top of the head. The alveolar 



