Fronds free floating on or beneath water surface, 1 or 2 coherent together, green on upper surface, green to transparent on lower surface, never reddish, symmetric, globose, ovoid, or boat-shaped, with 1 basal cavity reproducing daughter fronds, cavity not surrounded by a scale at base; veins absent. Pigment cells (in our species) absent in vegetative cells. Roots absent. Daughter fronds connected to mother frond by a very short stipe (not visible). Flower emerging from a cavity near median line of upper surface, not surrounded by a scale; ovary with 1 ovule; stamen 1, 2-locular. Seed 1, subsmooth.

Eleven species: nearly cosmopolitan; one species in China.