5 2 4 



The True Sharks 



Family Hexanchidae. The majority of the living Notidanoid 

 sharks belong to the family of Hexanchidce. 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 in the warm seas. The two genera, Hexanchus, with 

 six, and Heptranchias, with seven vertebrae, are found in the 

 Mediterranean. The European species are Hexanchus griseus, 

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

 to the West Indies. In California, Heptranchias maculatus 



FIG. 313. Teeth of Heptranchias indicus Gmeliru 



and Hexanchus corinus are occasionally taken, while Heptran- 

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

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



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

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

 name than Hexanchus 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 

 of 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 



