The Hexanchiformes are the order consisting of the most primitive types of sharks , and numbering just six extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens.
Hexanchiform sharks have only one dorsal fin , either six or seven gill slits , and no nictitating membrane in the eyes. Shark teeth similar to those modern hexanchids are known from Devonian deposits in Antarctica and Australia , as well as Permian deposits in Japan . If these are in fact hexanchids, this may be the only extant order of elasmobranchs to have survived after the Permian extinction (and by extension, the oldest extant order of elasmobranchs) .
The frilled shark, Chlamydoselachus anguineus , is very different from the cow sharks , and it has been proposed to be moved to its own order, Chlamydoselachiformes.
==Classific Living
Family Chlamydoselachidae Garman 1884Chlamydoselachus Garman , 1884Chlamydoselachus africana Ebert & Compagno , 2009 (Southern African frilled shark )Chlamydoselachus anguineus Garman, 1884 (Frilled shark )Family Hexanchidae J. E. Gray 1851 (Cow sharks)Heptranchias Rafinesque , 1810Heptranchias perlo (Bonnaterre , 1788) (sharpnose sevengill shark)Hexanchus Rafinesque , 1810Hexanchus griseus(Bonnaterre , 1788) (bluntnose sixgill shark)Hexanchus nakamurai Teng, 1962 (bigeyed sixgill shark)Notorynchus Ayres , 1855Notorynchus cepedianus (Péron , 1807) (broadnose sevengill shark)Extinct species Family Chlamydoselachidae Chlamydoselachus Garman, 1884 Chlamydoselachus bracheri Pfeil, 1983 Chlamydoselachus fiedleri Pfeil, 1983 Chlamydoselachus garmani Welton, 1983 Chlamydoselachus goliath Antunes & Cappetta , 2002 Chlamydoselachus gracilis Antunes & Cappetta , 2002 Chlamydoselachus keyesi Mannering & Hiller, 2008 Chlamydoselachus landinii Carrillo-Briceño, Aguilera & Rodriguez, 2014 Chlamydoselachus lawleyi Davis, 1887 Chlamydoselachus tatere Consoli, 2008 Chlamydoselachus thomsoni Richter & Ward, 1990 Chlamydoselachus tobleri Leriche, 1929 Proteothrinax Pfeil, 1983 Proteothrinax baumgartneri Pfeil, 1983 Family Crassonotidae Crassodontidanus Kriwet & Klug, 2011 Crassodontidanus serratus Fraas, 1855 Crassodontidanus wiedenrothi Thies, 1983 Notidanoides Maisey, 1986 Notidanoides muensteri Agassiz , 1843Notidanus Cuvier , 1816Notidanus amalthei Oppel, 1854 Notidanus atrox Ameghino, 1899 Notidanus intermedius Wagner, 1862 Notidanus nikitini Chabakov & Zonov, 1935 Pachyhexanchus Cappetta, 1990 Pachyhexanchus pockrandti Ward & Thies, 1987 Family Hexanchidae Gladioserratus Underwood, Goswami, Prasad, Verma & Flynn, 2011 Gladioserratus aptiensis Pictet, 1864 Gladioserratus dentatus Guinot, Cappetta & Adnet, 2014 Gladioserratus magnus Underwood, Goswami, Prasad, Verma & Flynn, 2011 Heptranchias Rafinesque, 1810 Heptranchias ezoensis Applegate & Uyeno, 1968 Heptranchias howellii Reed, 1946 Heptranchias karagalensis Kozlov in Zhelezko & Kozlov, 1999 Heptranchias tenuidens Leriche, 1938 Hexanchus Rafinesque, 1810 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 , 1843Hexanchus tusbairicus Kozlov in Zhelezko & Kozlov, 1999 Notidanodon Cappetta, 1975 Notidanodon brotzeni Siverson, 1995 Notidanodon dentatus Woodward, 1886 Notidanodon lanceolatus Woodward, 1886 Notidanodon loozi Vincent, 1876 Notidanodon pectinatus Agassiz, 1843 Notorynchus Ayres , 1855Notorynchus 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 Cappetta, 1990 Welcommia bodeuri Cappetta, 1990 Welcommia cappettai Klug & Kriwet, 2010 Weltonia Ward, 1979 Weltonia ancistrodon Arambourg, 1952 Weltonia burnhamensis Ward, 1979 Family Mcmurdodontidae ?Mcmurdodus White, 1968 Mcmurdodus featherensis White, 1968 Mcmurdodus whitei Turner, & Young, 1987 Family Orthacodontidae Occitanodus Guinot, Cappetta & Adnet, 2014 Occitanodus sudrei Guinot, Cappetta & Adnet, 2014 Orthacodus Woodward, 1889 Orthacodus longidens Agassiz, 1843 Sphenodus Agassiz, 1843 Sphenodus alpinus Gümbel, 1861 Sphenodus longidens Agassiz, 1843 Sphenodus lundgreni Davis, 1890 Sphenodus macer Quenstedt, 1852 Sphenodus nitidus Wagner, 1862 Sphenodus planus Agassiz, 1843 Sphenodus rectidens Emmons, 1858 Sphenodus robustidens Seguenza, 1900 Sphenodus tithonius Gemmellaro, 1871 Sphenodus virgai Gemmellaro, 1871 
