MYRIOPHYLLUM, in botany, a genus of the monoccia polyandria class. The calix of both male and female consists of four leaves, and none of them have any corolla: the male has eight stamens, and the female four pistilla; and there are four naked seeds. There are two species, both natives of Britain, viz. the spicatum, or spiked water-mill-foil; and the verticillatum, or verticillated mill-foil.