THE SKULL. 489 



It is less important in the Sanropsi<]a, and becomes indistingiiish- 

 ably fused with the sphenoid in the adult, while in Mammalia it is 

 no longer found. 



Ossification of the Cartilag'inous Cranium, In certain Fishes 

 the cartilaginous cranium remains quite unossified, while completely 

 enveloped in dermal bones. Such for instance is its condition in 

 the Selachioid Ganoids, In most instances, however, the investment 

 of the cartilaginous cranium by membrane bones is accompanied by 

 a more or less complete ossification of the cartilage itself 



In the Dipnoi this occurs to the smallest extent, the only ossifica- 

 tions occurring in the lateral parts of the occipital region, and forming 

 the exoccipitals. 



In Teleostei and bony Ganoids, a considerably greater number of 

 ossifications occur in the cartilage. 



In the region of the occipital cartilaginous ring there appears a 

 basioccipital and supraoccipital and two exoccipitals. The 

 hasioccipital is the only bone on the floor of the skull ossifying that 

 part into which the notochord is primitively continued*. 



In the region of the periotic cartilage a large number of bones 

 may appear. In front there is the prootic, which often meets the 

 exoccipital behind ; behind there is above and in close connection 

 with the supraoccipital the epiotic, and below in close connection 

 with the exoccipital the opisthotic. On the dorsal side of the 

 cartilage there is a projecting ridge composed mainly of a bone known 

 as the pterotic, sometimes erroneously called the squamosal, and 

 continued in front by the sphenotic. The pterotic, or the cartilagi- 

 nous 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 comparatively 

 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 notochoifl appears also to enter into the posterior part of the region which 

 ossifies as the bas^isplienoid. 



