Perennial herbs. Rhizome subterranean, jointed, creeping, branching into aerial, erect, usually annual stems. Stems hollow, grooved, simple or verticillately branched. Leaves whorled, scale-like, usually not green, united into a toothed or lobed sheath. Reproductive organs in terminal cones or spikes consisting of whorled, pedicellate, peltate, scale-like sporangiophores. Sporangia numerous on each sporangiophore, membranous, dehiscent by a longitudinal slit. Spores numerous, globular, with four hygroscopic spiral bands (elaters) attached at one point. Prothallia cushion-like at base, with green flat lobes on upper surface, usually dioecious, sometimes with archegonia and antheridia succeeding each other on the same prothallium.