xin PHYLUM CHORDATA 175 



cases are represented by a single basi-branchial plate (Fig. 835, 

 bas. br.). In the Rays the fifth branchial arch articulates with the 

 pectoral arch, a connection 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. 815, 835, 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 anterior 



ft. orb 



\ .,,., ,..., 



FIG. 836. Lateral view of the skull of Heptanchus. mck. Meckel's cartilage ; pal.qu. palato- 

 quadrate ; pt. orb. postorbital process of the cranium, with which the palatoquadrate 

 articulates. (After Gegenbaur.) 



vertebral plate of the spinal column by a distinct articulation, 

 the portion of the arch on which the articular surface is situated 

 sometimes forming an independent cartilage (supra-scapula). In 

 Heptanchus a small median ventral element may represent the 

 sternal apparatus of the Amphibia. 



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

 iii'-xo-, and meta-ptwygium (Figs. 815 and 835), 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. 835) into several seg- 

 ments, and the former articulates, through the intermediation of 

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

 skull. 



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

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



