Cow sharks are a family of sharks, the Hexanchidae, characterized by an additional pair or pairs of gill slits. They are considered the most primitive of all living sharks, as their skeletons resemble those of ancient extinct forms, with few modern adaptations. Cow sharks are ovoviviparous, with the mother retaining the egg cases in her body until they hatch. They feed on relatively large fish of all kinds, including other sharks, as well as on crustaceans and carrion.

Some key features of cow sharks include:

• Gill slits: Cow sharks have either six or seven pairs of gill slits, instead of the usual five, which links them to the sharks of the past.

• Body shape: Cow sharks have long and fairly slender bodies. Some species can grow to over 5m in length.

• Eyes: Two species have small eyes and two have large eyes. The Bluntnose Sixgill has eyes that glow a florescent blue-green.

• Teeth: The teeth differ in the upper and lower jaw. Those in the upper jaw being smaller, cusped, and more pointed than the large comb-like one in the lower jaw.

• Fins: Cow sharks have largish angular pectoral fins and smaller pelvic fins with a single anal fin present. They have only one dorsal fin with no spine. The caudal fin has a well-developed upper lobe with an obvious sub-terminal notch.

There are 37 species of cow shark, four of which are extant, in 10 genera. Cow sharks are found in Atlantic, Indian, and Pacific Oceans, usually in deep water.