J. Bot. (Hooker) 4: 421, plate 25, fig. A. 1842.
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.
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.