Subaquatic ferns. Rhizome long-creeping, filiform or thick and woody, clothed with hairs. Sterile fronds thread-like or with 2 or 4 leaflets at the apex of a long stalk; the vernation circinnate. Spores of 2 kinds; megaspores single in each megasporangium; microspores numerous in each microsporangium. Sporangia numerous in sessile or stalked, usually globose to oblong sporocarps. 3 gen., trop. to temp.