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Cow sharks are a shark family, the Hexanchidae, 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 .
Due to their primitive anatomy, hexanchiforms were previously considered the most basal group of sharks. However, more recent phylogenetic studies indicate that while primitive, they in fact belong to the superorder Squalomorphii, which also contains dogfishes, angelsharks, and sawsharks, although they are thought to be the most basal member of the group.
The bluntnose sixgill shark (Hexanchus griseus), often simply called the cow shark, is the largest hexanchoid shark, growing to 20 ft (6.1 m) in length. [2] It is found in tropical and temperate waters worldwide and its diet is widely varied by region.
The bigeyed sixgill shark (Hexanchus nakamurai) is a cow shark of the family Hexanchidae. Its dorsal surface has a brownish-gray color, and is sharply separated from the light coloring of its ventral surface. The eyes are a fluorescent green while the shark is alive. The body of this shark is small, slim, and fusiform in shape.
The sixgill sharks are a genus, Hexanchus, of deepwater sharks in the family Hexanchidae.These sharks are characterized by a broad, pointed head, six pairs of gill slits, comb-like, yellow lower teeth, and a long tail.
The shark order Hexanchiformes (in a broad sense, not only comprising the cow sharks, but also including the frilled sharks, Chlamydoselachidae) is often considered the most primitive of extant sharks, since they share some features with Paleozoic and early-Mesozoic shark groups as the Cladoselachiformes.
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