Plants: small, in loose mats, dull to bright green. Stems: creeping, densely terete-foliate, occasionally subsecund, irregularly branched, branches densely foliate; central strand absent; pseudoparaphyllia narrowly acute; axillary hairs of 4–6 cells. Stem: leaves appressed when dry, spreading when moist, imbricate, ovate to ovate-lanceolate, concave, not plicate, 0.4–0.7 mm; base scarcely decurrent; margins entire, subentire to serrulate distally; apex acute to narrowly obtuse; costa to 33–66% leaf length, slender, terminal spine absent; alar cells subquadrate to oblate-quadrate in several rows, little different from more distal cells; laminal cells rhombic to oblong-rhombic, 1.5–3:1, walls thin; proximal cells quadrate to subquadrate, shorter at margins. Branch: leaves similar. Sexual: condition autoicous; perichaetial leaves erect to flexuose, apex abruptly acuminate. Seta: reddish, smooth. Capsule: erect, brown, narrowly elliptic to oblong, symmetric; annulus not separating; operculum long-rostrate; peristome without apparent hygroscopic movement, modified; exostome teeth very short, 1/2 endostome length. Calyptra: naked. Spores: 14–21 µm. c, e United States, Europe.
Species 1. Clasmatodon has been variously placed in Fabroniaceae or Myriniaceae, but molecular data strongly suggest a position in Brachytheciaceae as a small, epiphytic, monospecific genus with a reduced peristome.