Cow sharks are a family, the Hexanchidae, of sharks characterized by an additional pair or pairs of gill slits. Its 37 species are placed within the 10 genera Gladioserratus, Heptranchias, Hexanchus, Notidanodon, Notorynchus, Pachyhexanchus, Paraheptranchias, Pseudonotidanus, Welcommia, and Weltonia. Their most distinctive feature, however, is the presence of a sixth, and, in two genera, a seventh, gill slit, in contrast to the five found in all other sharks. They range from 1.4 to 5.5 m (4.6 to 18.0 ft) in adult body length.