Plants of wet places, sometimes submerged but more commonly above water level, in carpets, cushions, or hummocks; stems sparsely forked, consisting of a central parenchyma surrounded by a wood cylinder of thick-walled prosenchyma enclosed by a cortex of 1 or more layers of large, empty hyaline cells; rhizoids restricted to the base of young gametophores, with oblique cross-walls. Stem leaves less crowded, usually differentiated in size, shape, and structure, the hyaline cells often divided, generally with a poor development of pores and fibrils but usually extensively resorbed on 1 or both surfaces and showing membrane pleats or irregular membrane gaps. Branches in fascicles spirally disposed around the stem and crowded at the stem tip in a headlike tuft, or capitulum, some branches spreading to ascending, others more slender and pendent; branch cortex usually of 1 layer of hyaline cells, often with some cells differentiated as enlarged, apically porose retort cells. Branch leaves arranged in 5 rows, sometimes conspicuously so, commonly bordered by a few rows of linear cells, otherwise consisting of a network of narrow green cells enclosing large, empty hyaline cells nearly always reinforced by ringlike fibrils and porose at the surface. Antheridial branches catkinlike, with leaves commonly crowded in spiral rows and often highly colored, the antheridia globose and long-stalked, borne singly beside each leaf of the catkin. Perichaetial branches short, bearing 1–5 flask-shaped archegonia at the apex, the perichaetial leaves much larger and otherwise differentiated from stem and branch leaves, enclosing the sporophyte until its maturity. Sporophyte consisting of a globose capsule and a massive foot embedded in the tip of the perichaetial branch, which elongates to elevate the mature capsule beyond the perichaetium; capsules eperistomate, operculate, the wall of the urn of several cell layers with no intercellular spaces, usually bearing in the lower half or more a large number of pseudostomata sunken beneath the surface and consisting of paired guard cells but no pore; columella domelike, surrounded and overarched by large, tetrahedral spores. Calyptrae of a delicate, hyaline membrane irregularly ruptured as the capsule nears maturity. Protonemata thallose, producing 1 or rarely 2 leafy gametophores, also proliferating secondary protonemata at the ends of marginal, filamentous outgrowths.
The family consists of a single genus.