170 ZOOLOGY SECT. 



Section ft. Eajida. 



Enselachii with dorso- ventral ly compressed body, and, usually, 

 feebly developed caudal fin. The pectorals are of great size, the 

 pelvics usually small. A ventral fin is usually absent. The verte- 

 brae of the anterior region are fused together. The branchial 

 apertures are ventral, the spiracles dorsal. 



This section comprises all the recent and extinct Rays (Skates, 

 Thorn-backs, Sting- Rays, Electric Rays, Saw-fish Rays). 



3. GENERAL ORGANISATION. 



External Characters. In general shape most Sharks 

 (Fig. 831) are somewhat fusiform and slightly compressed 

 laterally. In the Rays (Fig. 832), on the other hand, there is 

 great dorso-ventral compression. The head is in many cases 

 produced forwards into a long rostrum, which is of immense 



F-io. 831. Porbeagle Shark (lamna cornubica). (From Dean's Fishes.) 



length and bordered with triangular teeth in -the Saw-fish Shark 

 (Pristiophorus) and Saw-fish Ray (Pristis). In the Hammerhead 

 Shark (Sphyrna or Zygcena} the anterior part of the head is 

 elongated transversely. 



There are well-developed median and paired fins. The caudal 

 fin is well developed, and, as a rule, strongly heterocercal in the 

 Sharks and shark-like Rays, feebly developed in most of the 

 latter group. The dorsal and ventral fins are large in the Sharks, 

 the former completely divided into two : in the Rays the dorsal 

 fin is usually small, and the ventral absent. The paired fins are 

 very differently developed in the two groups. In the Sharks both 

 pairs are well developed, the pectoral being the larger. In the 

 Rays the pectoral fins are extremely large, very much larger than 

 the pelvic, fringing the greater part of the length of the flattened 

 body, and becoming prolonged forwards on either side and even in 

 front of the head, so that the animal presents the appearance of a 

 broad fleshy leaf. 



In all recent Elasmobranchs the male has, connected with the 



