Plants tufted, grasslike, heterosporous (megaspores and microspores not alike), perennial; evergreen aquatics to ephemeral terrestrials. Rootstock brown, cormlike, lobed. Roots arising along central groove separating each rootstock lobe, simple or dichotomously branched, containing eccentric vascular strand and surrounding lacuna. Leaves linear, simple, spirally or distichously arranged, dilated toward base, tapering to apex, containing 4 transversely septate longitudinal lacunae, a central collateral vascular strand, and frequently several peripheral fibrous bundles; ligule inserted above sporangium. Megasporophylls and microsporophylls usually borne in alternating cycles; hardened scales and phyllopodia occasionally surrounding leaves. Sporangia solitary, adaxial, embedded in basal cavity of leaf, velum (thin flap extending downward over sporangium) partly to completely covering adaxial surface of sporangium; megasporangium with several to hundreds of megaspores; microsporangium with thousands of microspores. Megagametophytes white, endosporic, exposed when megaspore opens along proximal ridges; archegonia 1 to several, indicated by quartets of brownish neck cells. Microgametophytes 9-celled, endosporic, antheridium releasing 4 multitailed spermatozoids.
Genus 1, species ca. 150 (24 species in the flora).