The “Sting-Rays” are as numerous as the Rays proper, but they inhabit rather tropical than temperate seas. The species armed with a spine use it as a weapon of defence, and the wounds inflicted by it are, to man, extremely painful, and have frequently occasioned the loss of a limb. We have mentioned above (p. 190) that the danger arises from the lacerated nature of the wound as well as from the poisonous property of the mucus inoculated. The spines (Fig. 98, p. 190) are always barbed on the sides, and may be eight or nine inches long in the larger species. They are shed from time to time, and replaced by others growing behind the one in function, as the teeth of the fishes of this order, or as the fangs of a poisonous snake. Fossil species of _Trygon_ and _Urolophus_ occur in the tertiary strata of Monte Bolca and Monte Postale. The genera into which the various species have been divided are the following:-- UROGYMNUS.--Tail long, without fin or spine, sometimes with a narrow cutaneous fold below. Body densely covered with osseous tubercles. Teeth flattened. 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. [Illustration: 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. SIXTH FAMILY--MYLIOBATIDÆ. _The disk is very broad, in consequence of the great development of the pectoral fins, which, however, leave the sides of the head free, and reappear at the extremity of the snout as a pair of detached (cephalic) fins. Viviparous._