xm PHYLUM CHORDATA 165 



together. The branchial apertures and the spiracle are situated 

 laterally. 



This section comprises all the recent Sharks and Dog-fishes, with 

 the exception of the Protoselachii. 



Section ft. Rajida. 



Euselachii with dorso-ventrally compressed body, and, usually-, 

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

 pelvics generally small. A ventral fin is usually absent. The 

 vertebrae 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. 

 842) are somewhat fusiform and slightly compressed laterally. 

 In the Rays (Fig. 843), on the other hand, there is great dorso- 

 ventral compression. The head is in many cases produced forwards 



FIG. 842. Porbeagle Shark (Lamna cornubica). (From Dean's Fishes.) 



into a long rostrum, which is of immense length and bordered with 

 triangular teeth in the Saw-fish Shark (Pristiophorus) and Saw-fish 

 Ray (Pristis). In the Hammerhead Shark (Spliijrna or Zygcena) the 

 anterior part of the head is elongated transversely. 



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

 fin is large, and, as a rule, strongly heterocercal in the Sharks and 

 shark-like Rays, reduced 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 differ widely 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. 



