128 



STRUCTURE OF THE VERTEBRATES 



developed, supporting the anterior projection of the head; and 

 there are a few minor specialized" structures. The great advance 

 which the early elasmobranchs made was in the gill, or visceral, 

 arches. The first gill arch developed into jaws (upper and lower), 

 and the second into the hyoid apparatus which functions in the 

 dogfish as a suspensory cartilage for the jaws and a support for 

 the throat and tongue. The suspensory cartilage is the hyomandih- 

 ular. It is attached to the chondrocranium at the otic capsule, 

 and passes laterally and slightly ventrally to the upper jaw. The 

 spiracle of the dogfish is posterior to it. The upper jaw cartilage 

 is the ptery go-quadrate, and the lower jaw is MeckeVs cartilage. 



^Rostrum 



Olfactory capsule 

 Ptery go-qua(^r ate 

 Meckel's cartilage 



Otic capsule 



Myomandihidar 



Branchial arches 



Fig. 64. Chondrocranium and Branchial Arches of the Dogfish. The jaws 

 and hyoid apparatus are shown in position. 



To summarize, the jaw apparatus of the elasmobranch is made 

 up of three major cartilages: (1) the suspensory hyomandibular, 

 (2) the ptery go-quadrate of the upper jaw, and (3) aNIeckel's 

 cartilage of the lower. 



The chondrostean skull (page 40) is useful in showing the 

 probable evolution of the higher skull. The chondrocranium is 

 like that of the dogfish, but the cartilages have become en- 

 cased in bone. The upper jaw became immovably' attached to the 

 skull. The lower jaw was covered with the bony plates, except at 

 the point of articulation with the upper jaw. The articulation re- 

 mained between the pterygo-quadrate and jNIeckel's cartilages. 



In the higher vertebrates the skull, as such, takes form. The 

 chondrocranium becomes ossified, or replaced by bone tissue, and 

 the dermal roofing bones fuse with the replacement bones of the 



