Plants: medium-sized to large, in loose tufts or extensive mats, deep green, light green, or whitish, brownish or paler green with age. Stems: creeping, terete- to subcomplanate-foliate, not julaceous, irregularly branched, branches terete-, complanate-, or subcomplanate-foliate; central strand present; pseudoparaphyllia acute or acuminate; axillary hairs of 3–7 cells, cells long, 3–6:1. Stem: leaves erectopatent or erect, gradually reflexed from erect base, imbricate to somewhat spaced, broadly ovate or suborbicular to ovate-lanceolate, flat to slightly concave, not or slightly plicate; base with decurrency absent after leaf is detached; margins serrate to serrulate; apex acuminate or ± broadly acute; costa to 40–85% leaf length, moderate to somewhat stout, terminal abaxial spine present or absent; alar cells poorly differentiated, or short-rectangular, slightly enlarged, region inconspicuous; laminal cells linear, walls moderately thick; basal cells somewhat shorter. Branch: leaves similar, smaller, sometimes narrower. Sexual: condition autoicous; perichaetial leaves abruptly contracted, acumen straight to reflexed. Seta: red-brown, smooth. Capsule: inclined to horizontal, red-brown to brown, oblong-cylindric, weakly curved; annulus separating; operculum rostrate; peristome xerocastique, perfect. Calyptra: naked. Spores: 9–16 µm. Nearly worldwide, tropical to north temperate regions, with a few boreal taxa.
Species 30–60 (2 in the flora). Aquatic plants of Rhynchostegium were often segregated as the separate genus Platyhypnidium, placed in Amblystegiaceae by M. Fleischer (1900–1922[–1923], vol. 4) and V. F. Brotherus (1924–1925). The latter genus was subsequently accepted by many bryologists, although some authors (such as J. PodpÄra 1954; N. Takaki 1956; H. Robinson 1962) included it in Rhynchostegium. Phylogenetic analyses by S. Huttunen and M. S. Ignatov (2010) found that aquatic species within Rhynchostegium originated several times, thus the acceptance here of Rhynchostegium in a broad sense. Robinson (1987) recognized the tropical species as distinct from the European R. confertum (Dickson) Schimper, the type species of the genus, and he segregated tropical taxa into Steerecleus. However, the broader analysis by Huttunen and Ignatov demonstrated that species with Steerecleus morphology are not monophyletic, so acceptance of this genus would cause severe splitting of Rhynchostegium and enormous nomenclatural problems. The North American species represent quite morphologically different lineages, Steerecleus and Platyhypnidium.