Fig. 312.—Griset or Cow-shark, Hexanchus griseus (Gmelin). Currituck Inlet, N. C.
The few living forms are of high interest. The extinct species are numerous, but not very different from the living species.
Family Hexanchidæ.—The majority of the living Notidanoid sharks belong to the family of Hexanchidæ. 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 postorbital 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 vertebræ, 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 and Hexanchus corinus are occasionally taken, while Heptranchias deani is the well known Aburazame or oil shark of Japan. Heptranchias indicus, a similar species, is found in India.
Fig. 313.—Teeth of Heptranchias indicus Gmelin.
Fossil Hexanchidæ exist in large numbers, all of them referred by Woodward to the genus Notidanus (which is a later name than Hexanchus and Heptranchias and intended to include 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 younger than the Cestraciontes and little older than the Lamnoids, or the Squaloid groups. Heptranchias microdon is common in English Cretaceous rocks, and Heptranchias primigenius and other species are found in the Eocene.
Family Chlamydoselachidæ.—Very great interest is attached to the recent discovery by Samuel Garman of the frilled shark, Chlamydoselachus anguineus, the sole living representative of the Chlamydoselachidæ.
