1 62 



SELACHII 



while the hyoid copula has disappeared, the basal elements of the 

 branchial arches fuse to a single large plate. But the hypobranchials are 

 more distinct than in the Rajidae. The ceratohyal is articulated to the 

 hyomandibular (Torpedo), or to the first branchial arch (Hypnos). The 

 jaws become very slender and crescentic, and bear small teeth. 





Fie. 125. 



Ventral view of the skull and visceral arches of Torpedo. (After Gegenbaur.) ao, enlarged 

 antorbital process ; h, fused basibranchials ; c, spiracular cartilage ; hrn, hyomandibular ; 

 hy, hyoid arch: m.k, Meckel's cartilage; n.c, nasal cartilage ; r, rostral process; s, base of 

 cranium ; 1-4, first four branchial arches ; 5, fifth branchial arch, which abuts against the 

 pectoral arch. 



The two halves of the pectoral girdle are not firmly fused ventrally, 

 neither do they articulate as in the Rajidae with the vertebral column, 

 but join above it to a median piece. The pelvic girdle has paired pre- 

 pubic processes. The ribs are well developed. A large electric organ, 

 supplied by a branch of the facial, and by four branches of the vagus 

 group of nerves, is situated between the branchial pouches and the 

 pectoral fin, on each side of the head (Fig. 126). This organ, formed of 

 vertical prismatic columns of tissue, is probably a greatly enlarged and 



