xiii PHYLUM CHORDATA 157 



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 the Sharks (Fig. 784), 

 for the most part, are somewhat fusiform and slightly com- 

 pressed laterally. In the Rays (Fig. 785), on the other hand, there is 

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

 duced forwards into a long rostrum, which is of immense length 

 and bordered with triangular teeth in the Saw-fish Shark (Pristio- 

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

 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 



FIG. 784. Shark (Lamna cornubica). (From Dean's Fishes.) 



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 

 pelvic fins, a pair of grooved appendages the claspers or pterygo- 

 podia which subserve copulation. 



The mouth is situated on the ventral surface of the head, usually 

 a considerable distance from the anterior extremity. In front of 



