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Sharks constitute 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.

Sharks are split into eight orders :

• Carcharhiniformes :   ground sharks , the largest order of sharks, including requiem sharks , catsharks , swellsharks , houndsharks , weasel sharks and hammerheads
• Heterodontiformes :   bullhead sharks
• Hexanchiformes :   cow sharks and frilled sharks
• Lamniformes :   mackerel sharks , including thresher sharks
• Orectolobiformes :   carpetsharks
• Pristiophoriformes :   sawsharks
• Squaliformes :   bramble sharks , dogfish , sleeper sharks and lanternsharks
• Squatiniformes :   angelsharks