Aquatic herbs, with opposite or verticillate pinnatifid submerged leaves, and clustered rhombic-ovate dentate floating ones with inflated petioles. Flowers perfect, axillary, solitary, short-peduncled. Calyx-tube short, the limb 4-parted, the lobes persistent. Petals 4, sessile, inserted on the perigynous disk. Stamens 4, inserted with the petals; filaments subulate-filiform. Ovary 2-celled, conic above; style subulate; ovule 1 in each cavity. Fruit coriaceous or bony, large, 2-4-spinose or swollen at the middle, 1-celled, 1-seeded, indehiscent. Seed inverted.

A single genus of 3 species, natives of Europe, tropical Asia and Africa.