Small aquatic or subaquatic plants, often persisting in seasonally dry areas. Rhizome creeping. Fronds simple and grass-like or with 4 (rarely 2) opposite leaflets. Sporangia borne in nut-like structure (sporocarp) attached near base of petiole or in leaf axil. Sporocarps sessile or pedunculate, oblong to globose; megasporangia and microsporangia in same sporocarp. Sporangia extruded in gelatinous mucilage when sporocarp ruptures. Megaspore solitary in megasporangium; microspores 32 or 64 in microsporangium. Gametophyte simple-few sperm produced within microspore and an egg cell within a few specialized cells of the megaspore, dioecious.
Cosmopolitan, 3 genera with about 70 species; 2 genera in Australia, both in Victoria.