xii PHYLUM CHORDATA 387 



going on. By the time the young fish is ready to be born 

 or hatched, the greater part of the yolk-sac has been drawn 

 into the ccelom, a mere vestige of it still dangling from the 

 ventral surface of the body. 



In all the most important features of their organisation 

 there is a considerable degree of uniformity among the 

 Elasmobranchii. 



In general shape the sharks (Fig. 231), for the most part, 

 are somewhat fusiform and slightly compressed laterally. In 

 the rays (Fig. 232), on the other hand, there is great dorso- 



Fig. 231. — Shark (Lamna cornubica). (From Dean's Fishes.) 



ventral compression. The head is in many cases produced 

 forwards 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 hammer- 

 head shark the anterior part of the head is elongated trans- 

 versely. 



There are well-developed median and paired fins. The 

 caudal fin is well developed, and, as a rule, strongly hetero- 

 cercal 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 smjill, and the ven- 



