59 6 OSSIFICATIONS OF THE CARTILAGINOUS CRANIUM. 



called the squamosal, and continued in front by the sphenotic. 

 The pterotic, or the cartilaginous region corresponding to it, 

 always supplies the articular surface for the hyomandibular. 



In the floor of the skull, in the region of the pituitary body, 

 there is formed a basisphenoid; while in the lateral parts of the 

 wall of this part of the cranium, there is a bone known as the 

 alisphenoid. 



In front, parts of the lateral walls of the cranium ossify as the 

 orbitosphenoids. 



In view of the very imperfect ossification of the cartilaginous 

 cranium of the Dipnoi, and of the fact that there is certainly no 

 direct genetic connection between the Teleostei on the one hand, 

 and the Amphibia and Amniota on the other, it is very difficult 

 to believe that most of the ossifications of the cranium in the 

 Amphibia and Amniota have more than a general correspondence 

 with those in the Teleostei. 



In the Amphibia the ossifications in the cartilage are compa- 

 ratively few. In the occipital region there is a lateral ossification 

 on each side of the exoccipital. the basioccipital region being 

 unossified, and the supraoccipital at the utmost indurated by a 

 calcareous deposit. 



The periotic capsule is ossified by a prootic centre, which 

 meets the exoccipital behind. 



The front part of the cartilaginous cranium is ossified by a 

 complete ring of bone the sphenethmoid bone which embraces 

 part of the ethmoid region, and of the orbitosphenoid and 

 presphenoid regions. 



In the Amphibia the cartilaginous cranium, with its centres 

 of ossification, is easily separable from the membranous investing 

 bones. 



In the Amniota the cartilaginous cranium, whose development 

 in the embryo has already been described, becomes in the adult 

 much more largely ossified, and the bones which replace the 

 primitive cartilage unite with the membrane bones to form a 

 continuous bony cranium. 



The centres of ossification become again much more numerous. 

 In the occipital segment analogous centres to those of Teleostei 

 are again found ; and it is probable that the exoccipitals are 

 homologous throughout the series, the supraoccipital and basioc- 



