THE SKULL. 595 



is usually dentigerous. The splenial membrane bone is the 

 most important bone on the inner side of Meckel's cartilage, but 

 other elements known as the coronoid and angular may also be 

 added. In Mammalia the dentary is the only element present 

 (vide p. 590). 



On the roof of the mouth a median bone, the parasphenoid, 

 is very widely present in the Amphibia and Fishes, except the 

 Elasmobranchii and Cyclostomata, and has no doubt the same 

 phylogenetic origin as the vomer and membranous palatines and 

 pterygoids. 



It is less important in the Sauropsida, and becomes indis- 

 tinguishably fused with the sphenoid in the adult, while in 

 Mammalia it is no longer found. 



Ossification of the Cartilaginous 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 

 ossifications 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 basioccipital is the only bone on the floor of the skull 

 ossifying that part into Avhich the notochord is primitively con- 

 tinued 1 . 



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 



1 The notochord appears also to enter into the posterior part of the region which 

 ossifies as the basisphenoid. 



382 



