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Phallostethidae (priapum fish; subclass Actinopterygii, order Atheriniformes) A family of very small, brackish-water and freshwater fish that have a relatively large, fleshy appendage (the priapum) below the head. The male uses the priapum during mating to secure internal fertilization of the eggs of the female. Almost transparent, Phenacostethus smithi (2–3 cm) is found in small schools in the rivers of Thailand. There are possibly four species, found in south-east Asia.