162 ZOOLOGY SECT. 



There are always five pairs of branchial arches except in 

 Hexanchus and Chlamydoselachus, which have six, and Heptanchus, 

 in which there are seven. Their dorsal ends are free in the 

 Sharks, articulated with the anterior vertebral plate of the spinal 

 column in most Rays. Externally they bear a series of slender 

 cartilaginous branchial rays. The median ventral elements of the 

 branchial arches are usually more or less reduced, and in some 

 cases are represented by a single basi-branchial plate (Fig. 788, 

 las. 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 the extra-branchial cartilages absent as 

 such in the Dog-fishes and Rays, support the branchial apertures. 



The pectoral arch (Figs. 770, 788, pcct.} 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, or scapular, and a ventral, or 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 

 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 and higher Vertebrata, 



The bascijlpterygiophores of the pectoral fin are typically three, pro-, 

 meso-, and meta-pterygium (Figs. 770 and 788), but there are some- 

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

 meta-pterygia are in the Rays divided (Fig. 788) into several seg- 

 ments, and the former articulates, through the intermediation of a 

 cartilage termed the ant-orbital ,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. 

 In some cases a median epipubic process projects forwards from 

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

 pubic process. A lateral iliac process which becomes highly 

 developed in the Holocephali is sometimes represented, -and may 

 attain considerable dimensions. The pelvic fin has usually two 

 basal cartilages, representing the pro- and meta-pterygia, but the 

 former is often absent. In the males special cartilages attached 

 to the meta-pterygia support the claspers. With the basal car- 

 tilages of both pectoral and pelvic fins are connected a number of 

 jointed cartilaginous fin-rays supporting the expanse of the fin. 



The arrangement of the muscles is simple. The trunk muscles 

 are divided into a pair of dorsal and a pair of ventral divisions, 

 each composed of many myomeres with intercalated myocommata 

 (Fig. 714, p. 63), following a metameric arrangement. The ventral 

 part, where it forms the muscle of the wall of the abdominal cavity, 



