An order of Squalomorpha that are known as the six-gill sharks. These sharks are distinguished by a combination of the following characters: six or seven gill slits, all anterior to the pectoral fins; a single dorsal fin without a spine; anal fin present (the only order of squalomorphs with an anal fin); the dorsal, anal, and pelvic fins placed far posteriorly; eyes lacking a nictitating fold or membrane (at the inner angle of the eye or below the eyelid that is capable of extending over the eyeball); spiracles present, but small and well behind the eyes; and ovoviviparous development. They comprise two extant families (chlamydoselachidae and Hexanchidae), four genera, and five species.