J. E. Gray, 1851
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.
Cow sharks are considered the most primitive of all the sharks, as their skeletons resemble those of ancient extinct forms, with few modern adaptations. Their excretory and digestive systems are also unspecialised, suggesting they may resemble those of primitive shark ancestors. A possible hexanchid tooth is known from the Permian of Japan, making the family a possible extant survivor of the Permian-Triassic extinction.
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.
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.
The only fossil records of the Cow Shark consist of mainly only isolated teeth. Although, skeletal remains for this species have been found from the Jurassic time period have been very rare and have only been found in the "Late Jurassic Lithographic Limestones of South Germany, Nusplingen, Solnhofen, and Late Cretaceous calcareous sediments of Lebanon.” These records have made scientists conclude that the Cow Shark is now a more "Diverse and numerous species". 
The 37 species of cow shark, in 10 genera, are:
- †Gladioserratus Underwood, Goswami, Prasad, Verma & Flynn, 2011
- Heptranchias Rafinesque, 1810
- Hexanchus Rafinesque, 1810
- Hexanchus griseus (Bonnaterre, 1788) (bluntnose sixgill shark)
- Hexanchus nakamurai Teng, 1962 (bigeyed sixgill shark)
- Hexanchus vitulus Daly-Engel, 2018
- †Hexanchus agassizi Cappetta, 1976
- †Hexanchus andersoni Jordan, 1907
- †Hexanchus casieri Kozlov, 1999
- †Hexanchus collinsonae Ward, 1979
- †Hexanchus gracilis Davis, 1887
- †Hexanchus hookeri Ward, 1979
- †Hexanchus microdon Agassiz, 1843
- †Hexanchus tusbairicus Kozlov in Zhelezko & Kozlov, 1999
- †Notidanodon Cappetta, 1975
- Notorynchus Ayres, 1855
- Notorynchus cepedianus (Péron, 1807) (broadnose sevengill shark)
- †Notorynchus borealus Jordan & Hannibal, 1923
- †Notorynchus kempi Ward, 1979
- †Notorynchus lawleyi Cigala Fulgosi, 1983
- †Notorynchus primigenius Agassiz, 1843
- †Notorynchus serratissimus Agassiz, 1843
- †Notorynchus subrecurvus Oppenheimer, 1907
- †Pachyhexanchus Cappetta, 1990
- †Pachyhexanchus pockrandti Ward & Thies, 1987
- †Paraheptranchias Pfeil, 1981
- †Paraheptranchias repens Probst, 1879
- †Pseudonotidanus Underwood & Ward, 2004
- †Pseudonotidanus semirugosus Underwood & Ward, 2004
- †Welcommia Klug & Kriwet, 2010
- †Weltonia Ward, 1979
- Froese, Rainer, and Daniel Pauly, eds. (2011). "Hexanchidae" in FishBase. February 2011 version.
- Allen, 45
- Compagno, L., Dando, M. and Fowler, S. Sharks of the World. Princeton Field Guides ISBN 0-691-12072-2
- "New information on the Devonian shark Mcmurdodus, based on material from western Queensland, Australia". www.academia.edu. Retrieved 2016-06-03.
- Matt's, J. & Last P.R. (1998). Paxton, J.R. & Eschmeyer, W.N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. p. 61. ISBN 0-12-547665-5.
- Allen, Thomas B. The Shark Almanac. New York: The Lyons Press, 1999. ISBN 1-55821-582-4
- Klug, Stefanie and Kruiwet, Jurgen, “A new Jurassic cow shark (Chondrichthyes, Hexanchiformes) with comments on Jurassic hexanchiform systematics” Swiss Journal of Geosciences, 12 November 2011. https://www.researchgate.net/publication/234060567_A_new_Jurassic_cow_shark_Chondrichthyes_Hexanchiformes_with_comments_on_Jurassic_hexanchiform_systematics