Classification

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks.

There are more than 360 described species of sharks.

There are eight orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:
Hexanchiformes: Examples from this group include the cow sharks, frilled shark and even a shark that looks on first inspection to be a marine snake.
Squaliformes: This group includes the bramble sharks, dogfish and roughsharks, and prickly shark.
Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they use for slashing the fish that they eat.
Squatiniformes: Also known as angel sharks, they are flattened sharks with a strong resemblance to stingrays and skates.
Heterodontiformes: They are generally referred to as the bullhead or horn sharks.
Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the whale shark.

Carcharhiniformes: These are commonly referred to as the groundsharks, and some of the species include the blue, tiger, bull, reef and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
Lamniformes: They are commonly known as the mackerel sharks. They include the goblin shark, basking shark, megamouth shark, the thresher sharks, shortfin and longfin mako sharks, and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes include the extinct megalodon, Carcharodon megalodon.