524 



The True Sharks 



Family Hexanchidse.— The majority of the living Notidanoid 

 sharks belong to the family of HexanchidcB. These sharks have 

 six or seven gill-openings, one dorsal fin, and a relatively simple 

 organization. The bodies are moderately elongate, not eel- 

 shaped, and the palato-quadrate articulates with the post- 

 orbital part of the skull. The six or eight species are found 

 sparsely m the warm seas. The two genera, Hexanchus, with 

 six, and Heptranchias, with seven iertebr^e, are fotmd in the 

 Mediterranean. The European species are Hexanchus grisens, 

 the cow-shark, and Heptranchias cinereus. The former crosses 

 to the West Indies. In California, Heptranchias niaculatus 



Fig. 313. — Teeth of Heptranchias indicus Gmelin. 



and HexancJiits coriniis are occasionally taken, while Heptran- 

 chias dcani is the well known Aburazame or oil shark of Japan. 

 Heptranchias indicus, a similar species, is found in India. 



Fossil Hcxancliidcv exist in large numbers, all of them re- 

 ferred by Woodward to the genus Notidanus (which is a later 

 name than HexancJms and Heptranchias and intended to in- 

 clude both these genera), differing chiefly in the number of gill- 

 openings, a character not ascertainable in the fossils. None 

 'if these, however, appear before Cretaceous time, a fact which 

 may indicate that the simplicity of structure in Hexanchus and 

 Heptranchias is a result of degeneration and not altogether a 

 mark of primitive simplicity. The group is apparently much 



