What is "potamotrygonidae"

River stingrays or freshwater stingrays are Neotropical freshwater fishes of the Potamotrygonidae family in the order Myliobatiformes, one of the four orders of batoids, cartilaginous fishes related to sharks. River stingrays are found in rivers in South America draining into the Caribbean Sea and the Atlantic Ocean as far south as the River Plate in Argentina. Each river system has its own endemic stingrays. They are generally pale brown, variously mottled or speckled, have discs ranging from in diameter and venomous caudal stings. There are about twenty-eight species in four genera.