in botany, a genus of the monocia polyandria clasps. The calyx of the male is divided into many segments; it has no corolla; and the lamina are from 16 to 20. The calyx and corolla of the female are the same with those of the male; it has one pistil, no stylus, and one naked seed. There are two species, one of which, viz. the demerlum or horned pondweed, is a native of Britain.