THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 



107 



premaxilla 



maxilla 



5. Membrane bones added to the ventral surface of the skull. These are from the 

 anterior end, posteriorly, chiefly: wmers, palatines, ptery go-ids (Fig. 37). It should be stated 

 again that palatines and pterygoids are membrane bones in some animals and cartilage bones 

 in others. The parasphenoid is a membrane bone which in Amphibia typically forms the 

 chief bone on the ventral surface of the skull, but it does not persist in higher forms. These 

 membrane bones are situated ventral to the ethmoid and part of the sphenoid bones, which 

 they conceal from surface view, but the posterior part of the ventral side of the skull is com- 

 posed of cartilage bones, sphenoids and occipitals. 



6. Membrane bones added to the jaws, and other gill arches. 



a) Upper jaw: The upper jaw (pterygoquadrate cartilages) becomes incased in membrane 

 bones. The chief ones from the tip of the jaw posteriorly are, on each side: premaxilla, 

 maxilla, jugal (malar], quadratojugal, and 



squamosal (see Fig. 37). The upper jaw in 

 elasmobranchs is separate from the chon- 

 drocranium, to which it is generally at- 

 tached by ligaments, but in all of the land 

 vertebrates the upper jaw is inseparably 

 fused to the ventral side of the chondrocra- 

 nium and becomes an integral part of the 

 skull. 



b) Lower jaw: The lower jaw (Meckel's 

 cartilages) is similarly sheathed in mem- 

 brane bones, Meckel's cartilage often per- 

 sisting within them. The chief membrane 

 bones of the lower jaw are the denlary, the 



splenial, the angular, the sur angular (also quadratojugal -l- 



given as supra-angular}, the coronoid, the 



gonial, and two or three others occurring 



only in extinct forms. In the evolution of 



the lower jaw there has been a continual 



decrease in the number of membrane bones 



(see Fig. 38). 



c) Other gill arches: No membrane 

 bones occur in connection with the hyoid 



external nares 



palatine 



pterygoid 



quadrate 



squamosal 



FIG. 37. Diagram of the cartilage and mem- 

 brane bones of the upper jaw and floor of the 

 skull. Membrane bones blank; cartilage bones 

 stippled. The cartilage bones come from the ptery- 

 goquadrate cartilage. (Modified from Kingsley's 

 Comparative Anatomy of Vertebrates.) 



or other gill arches. 



With this account of the composition of the skull in mind we may now turn to the study 

 of some vertebrate skulls. 



E. THE SKULL OF NECTURUS, A PARTIALLY OSSIFIED SKULL 



In the skull of Amphibia the ossification of the chondrocranium has pro- 

 ceeded to but a limited extent so that a partially cartilaginous chondrocranium 

 is present from which the incasing membrane bones can be readily separated as 

 in fishes. The pterygoquadrate cartilages are inseparably fused to the ventral 

 and lateral sides of the skull proper and are partially ossified. In the lower 

 jaws Meckel's cartilages are persistent as cores inclosed by membrane bones. 

 The number of membrane bones in the skulls of present-day Amphibia is con- 

 siderably less than that of extinct forms, such as shown in Figure 



