Hexanchiformes

Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just six extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens. Hexanchiform sharks have only one dorsal fin, either six or seven gill slits, and no nictitating membrane in the eyes. For more information visit the Wikipedia Entry Get back to cartilaginous fishes

Frilled shark

Sevengill shark