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Potamotrygonidae (river stingray; subclass Elasmobranchii, order Rajiformes) A small family of freshwater stingrays in which the body has an almost circular profile ending in a fairly solid tail bearing a poisonous spine much feared by fishermen. These rays can be found partially buried in the mud of rivers. There are about 10 species, in S. America, W. Africa, and south-east Asia.