The True Sharks 185 
Family Hexanchide.—The majority of the living Notidanoid 
sharks belong to the family of Hexanciide. 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 vertebrx, 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 
Fic. 127.—Teeth of Heptranchias indicus Gmelin. 
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 Hexanchide 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 gil - 
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 
