Cow sharks, or the Hexanchidae, are a family of sharks characterized by an additional pair or pairs of gill slits. There are currently known to be four species in three genera, Heptranchias, Hexanchus and Notorynchus,. Cow sharks are considered the most primitive of all the sharks, because their skeletons resemble those of ancient extinct forms, with few modern adaptations. Their excretory and digestive systems are also unspecialised, suggesting that they may resemble those of primitive shark ancestors. Their most distinctive feature, however, is the presence of a sixth, and, in two genera, a seventh, gill slit, in addition to the five found in all other sharks. They range from 1.4 metres (4.6 ft) to over 5.5 metres (18 ft) in adult body length. 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.