The bullhead sharks are a small order (Heterodontiformes) of basal modern sharks (Neoselachii). There are nine living species in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small, with the largest species being just 150 centimetres (59 in) in adult length. They are bottom feeders in tropical and subtropical waters.
The Heterodontiforms appear in the fossil record in the Early Jurassic, well before any of the other Galeomorphii, a group which includes all modern sharks except the dogfish and its relatives. However, they have never been common, and it is likely their origin lies even further back.
The Heterodontiforms are morphologically rather distinctive. The mouth is located entirely anterior to the orbits. Labial cartilages are found in the most anterior part of the mouth. Nasoral grooves are present, connecting the external nares to the mouth. The nasal capsules are "trumpet-shaped" & well-separated from orbits. Circumnarial skin folds present; but the rostral process of neurocranium (braincase) absent, although a precerebral fossa is present. Finally, the braincase bears a supraorbital crest.
The eyes lack a nictitating membrane. A spiracle is present, but small. The dorsal ends of branchial arches 4 and 5 are attached, but not fused into a "pickaxe" as in lamniform sharks. Heterodontiforms have 2 dorsal fins, with fin spines, as well as an anal fin. The dorsal and anal fin also contain basal cartilages, not just fin rays.
There are nine living species of bullhead shark:
- Horn shark, Heterodontus francisci (Girard, 1855) 
- Crested bullhead shark, Heterodontus galeatus (Günther, 1870) 
- Japanese bullhead shark, Heterodontus japonicus Maclay & Macleay, 1884 
- Mexican hornshark, Heterodontus mexicanus Taylor & Castro-Aguirre, 1972 
- Oman bullhead shark, Heterodontus omanensis Baldwin, 2005
- Port Jackson shark, Heterodontus portusjacksoni (Meyer, 1793) 
- Galapagos bullhead shark, Heterodontus quoyi (Fréminville, 1840) 
- Whitespotted bullhead shark, Heterodontus ramalheira (Smith, 1949) 
- Zebra bullhead shark, Heterodontus zebra (Gray, 1831) 
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