Sp. Pl. 2: 1100. 1753; Gen. Pl. ed. 5, 486, 1754.
Plants aquatic, commonly entirely under water and infrequently persisting on bare mud. Roots arising at nodes. Leaves filiform, terete, lacking expanded blades. Sporocarps subterranean, borne on unbranched stalks arising at bases of petioles, attached laterally to stalk apex (attached portion called raphe), lacking teeth; sporocarps densely to sparsely hairy, less so with age, dehiscing into 4 valves.
North America, South America, Europe, Pacific Islands in New Zealand, Australia
Species 6 (1 in the flora).