170 ZOOLOGY SECT. 



tion which is absent in the Sharks. A series of slender cartilages, 

 probably modified branchial rays the extra-branchial cartilages 

 absent as such in some Dog-fishes and Rays, support the branchial 

 apertures. 



The pectoral arch (Figs. 825, 846, pect.) consists of a single 

 cartilage, with, however, in most of the Sharks, a mesial flexible 

 portion by which it is divided into right and left halves. Each 

 lateral half consists of a dorsal scapular and a ventral cora- 

 coid part, the two being separated by the articular surfaces for the 

 basal cartilages of the fin. In the Rays, but not in the Sharks, 

 the dorsal ends of the pectoral arch are connected with the spinal 

 column (anterior vertebral plate) by a distinct articulation, the por- 

 tion of the arch on which the articular surface is situated some- 

 times forming an independent cartilage (supra-scapula). In Hep- 



Fia. 847. Lateral view of the skull of Heptanchus. mck. Meckel's cartilage ; pal. qu. palato- 

 quadrate ; pt. orb. post-orbital process of the cranium, with which the palatoquadrate 

 articulates. (After Gegenbaur.) 



tanckus a small median ventral element may represent the sternal 

 apparatus of the Amphibia. 



The basal pterygiophores of the pectoral fin are typically three, pro-, 

 meso-, and meta-pterygium (Figs. 825 and 846), but there are some- 

 times four, and the number may be reduced to two. The pro- and 

 meta-pterygia are divided in the Rays (Fig. 846) into several seg- 

 ments, and the former articulates, through the intermediation of 

 a cartilage termed the antorbital, with the olfactory region of the 

 skuU. 



The pelvic arch (pi.) is usually, like the pectoral, a single cartilage, 

 but in some exceptional cases it consists of two lateral portions. 

 In some cases a median epipubic process projects forwards from 

 the pelvic arch, and frequently there is on each side a prepubic 



