Only one species is known (U. asperrimus), common in the Indian Ocean, and with a body from 4 to 5 feet long; the skin is frequently used for covering shields and the handles of swords and other weapons, its rough surface offering a firm hold to the hand.
Trygon.—Tail very long, tapering, armed with a long arrow-shaped barbed spine. Body smooth or with tubercles. Nasal valves coalescent into a quadrangular flap. Teeth flattened.
Some twenty-five species are known, one of which (T. pastinaca) extends from the south coast of England and the east coast of North America through the Atlantic and Indian Ocean to Japan. The majority of the species belong to the tropical parts of the Indian and Atlantic Oceans; some inhabit exclusively freshwaters of eastern tropical America. A closely allied genus is Tæniura, with six species.
Urolophus.—Tail of moderate length, with a distinct rayed terminal fin, armed with a barbed spine, without or with a rudimentary dorsal fin. Teeth flattened.
Seven species from tropical seas, apparently of small size.
Fig. 128.—Urolophus cruciatus, from Australia.
Pteroplatea.—Body at least twice as broad as long; tail very short and thin, without or with a rudimentary fin, and with a serrated spine. Teeth very small, uni- or tri-cuspid.
Six species from temperate and tropical seas.