Hexanchiformes is the order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 consisting of the most primitive types of shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, and numbering just five extant species. Fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 sharks that were apparently very similar to modern sevengill species are known from Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 specimens.

Hexanchiform sharks have only one dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

, either six or seven gill slit

Gill slit

Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of Cartilaginous fish such as sharks, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...

s, and no nictitating membrane

Nictitating membrane

The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...

 in the eyes.

The frilled shark, Chlamydoselachus anguineus, is very different from the cow shark

Cow shark

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,....

s, and it has been proposed that it be moved to its own order Chlamydoselachiformes.

Extant species

• Family Chlamydoselachidae Garman 1884 (frilled sharks)
  • Chlamydoselachus
    Chlamydoselachus
    Chlamydoselachus is a genus of sharks and the sole extant member of the family Chlamydoselachidae, in the order Hexanchiformes. It contains two extant and several extinct species.-Species:...
    
    Garman, 1884
    • Chlamydoselachus africana Ebert & Compagno
      Leonard Compagno
      Leonard Joseph Victor Compagno is an international authority on shark taxonomy and the author of many scientific papers and books on the subject, best known of which is his 1984 catalogue of shark species produced for the Food and Agriculture Organization of the United Nations.-Career:*Ph.D,...
      
      , 2009 (Southern African frilled shark)
    • Chlamydoselachus anguineus Garman, 1884 (Frilled shark)
• Family Hexanchidae J. E. Gray
  John Edward Gray
  John Edward Gray, FRS was a British zoologist. He was the elder brother of George Robert Gray and son of the pharmacologist and botanist Samuel Frederick Gray ....
  
   1851 (cow sharks)
  • Heptranchias Rafinesque, 1810
    • Heptranchias perlo (Bonnaterre
      Pierre Joseph Bonnaterre
      Abbé Pierre Joseph Bonnaterre was a French naturalist who contributed sections on cetaceans, mammals, birds, reptiles, amphibians, and insects to the Tableau encyclopédique et méthodique...
      
      , 1788) (Sharpnose sevengill shark)
  • Hexanchus
    Hexanchus
    The sixgill sharks, Hexanchus, are a genus of deepwater sharks in the family Hexanchidae. These sharks are characterized by broad pointed heads, six pairs of gill slits, comb-like, yellow lower teeth, and a long tail. It can grow up to 5 m long and weigh over 600 kg...
    
    Rafinesque, 1810
    • Hexanchus griseus  (Bonnaterre
      Pierre Joseph Bonnaterre
      Abbé Pierre Joseph Bonnaterre was a French naturalist who contributed sections on cetaceans, mammals, birds, reptiles, amphibians, and insects to the Tableau encyclopédique et méthodique...
      
      , 1788) (Bluntnose sixgill shark)
    • Hexanchus nakamurai Teng, 1962 (Bigeyed sixgill shark)
  • Notorynchus Ayres
    William Orville Ayres
    William Orville Ayres was an American physician and ichthyologist. Born in Connecticut, he studied to become a doctor at Yale University School of Medicine....
    
    , 1855
    • Notorynchus cepedianus  (Péron
      François Péron
      François Auguste Péron was a French naturalist and explorer. He is credited with the first use of the term anthropology.-Explorations:...
      
      , 1807) (Broadnose sevengill shark)

Extinct species

• Family Chlamydoselachidae
  • Chlamydoselachus Garman, 1884
    • Chlamydoselachus bracheri Pfeil, 1983
    • Chlamydoselachus gracilis Antunes & Cappetta, 2001
    • Chlamydoselachus goliath Antunes & Cappetta, 2001
    • Chlamydoselachus fiedleri Pfeil, 1983
    • Chlamydoselachus lawleyi Davis, 1887
    • Chlamydoselachus thomsoni Richter & Ward, 1990
    • Chlamydoselachus tobleri Leriche, 1929
  • Thrinax Pfeil, 1983
    • Thrinax baumgartneri
      Thrinax baumgartneri
      Thrinax baumgartneri is an extinct species of shark. It was classified based on two relatively complete teeth and two isolated crowns. Due to the limited material available and its similarities to Chlamydoselachus fiedleri, it has been suggested that the criteria with which T...
      
      Pfeil, 1983
• Family Hexanchidae
  • Heptranchias Rafinesque, 1810
    • Heptranchias ezoensis Applegate & Uyeno, 1968
    • Heptranchias howelii (Reed, 1946)
    • Heptranchias tenuidens (Leriche, 1938)
  • Hexanchus Rafinesque, 1810
    • Hexanchus arzoensis (Debeaumont, 1960)
    • Hexanchus agassizi http://www.sheppeyfossils.com/pages/hexanchus_agassizi.htm
    • Hexanchus collinsonae  Ward, 1979
    • Hexanchus gracilis  (Davis, 1887) http://home.tiscali.be/belgiansharkteeth/post_oligocene/sharks/hexachus.htm
    • Hexanchus griseus “andersoni” “gigas” (Bonaterre, 1788)
    • Hexanchus hookeri  Ward, 1979
    • Hexanchus microdon “agassizii” (Agassiz, 1843)
    • Hexanchus nakamurai “vitulus” Teng, 1962
  • Notidanoides Maisey 1986 http://www.paleodb.org/cgi-bin/bridge.pl?a=basicTaxonInfo&taxon_no=86134
    • (No named species)
  • Notidanodon Cappetta, 1975
    • Notidanodon antarcti Grande & Chatterjee, 1987
    • Notidanodon brotzeni Siverson, 1995
    • Notidanodon dentatus (Woodward, 1886)
    • Notidanodon lanceolatus (Woodward, 1886)
    • Notidanodon loozi (Vincent, 1876)
    • Notidanodon pectinatus (Agassiz, 1843)
  • Notorynchus Ayres
    William Orville Ayres
    William Orville Ayres was an American physician and ichthyologist. Born in Connecticut, he studied to become a doctor at Yale University School of Medicine....
    
    , 1855
    • Notorynchus aptiensis (Pictet, 1865)
    • Notorynchus intermedius Wagner
    • Notorynchus lawleyi Ciola & Fulgosi, 1983
    • Notorynchus munsteri (Agassiz, 1843)
    • Notorynchus serratissimus (Agassiz, 1844)
    • Notorynchus serratus (Agassiz, 1844)
  • Paraheptranchias PFEIL, 1981
    • Paraheptranchias repens (Probst, 1879)
    • Paranotidanus “Eonotidanus” contrarius (Munster, 1843)
    • Paranotidanus intermedius (Wagner, 1861)
    • Paranotidanus munsteri (Agassiz, 1843)
    • Paranotidanus serratus (Fraas, 1855)
  • Pseudonotidanus Underwood & Ward, 2004
    • Pseudonotidanus semirugosus Underwood & Ward, 2004
  • 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