Herb floating at water surface, solitary or 2 thalli remaining connected. Roots absent. Thallus symmetric, thick, ± globular, ellipsoid or ovoid, the greatest part of the thallus submerged, green; veins not visible; dorsal and ventral scale absent. Budding pouch with lateral (actually basal) slit. Inflorescence of 1 female and 1 male flower; bract absent; anther unilocular.
About 11 species, cosmopolitan, in temperate and tropical regions; 2 species in Victoria.