Prodr. Aethéogam., 75. 1805.
Plants (1–) 2–3 (–6) cm. Stems red or reddish-brown. Leaves green to dark green, contorted and twisted when dry, ovate-elliptic or sometimes elliptic or oblong-ovate, rarely narrow, (2–) 3–4 (–5) mm; base long-decurrent; margins reddish-brown, 2-stratose or rarely multistratose, toothed to below mid leaf or sometimes only distally, teeth paired or occasionally single, small to large, blunt or occasionally sharp; apex acute or acuminate, rarely obtuse or rounded, apiculate or cuspidate, cusp toothed; costa percurrent, subpercurrent, or excurrent, smooth, on well-developed leaves distal abaxial surface rarely with few blunt teeth; medial laminal cells ± isodiametric or short-elongate, (18–) 20–30 (–40) µm, slightly smaller towards margins, in diagonal or longitudinal rows, usually strongly collenchymatous, blue postmortal color absent; marginal cells linear, in 2–3 (–4) rows. Sexual condition synoicous or rarely dioicous. Seta single. Capsule yellowbrown, 2–4 mm; operculum rostrate; exostome reddish-brown, sometimes yellowish. Spores 18–32 µm.
Phenology: Capsules mature late spring.
Habitat: Forests on humus, soil, logs, rock, base of trees, shaded cliffs
Elevation: low to moderate elevations
Greenland, Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que., Sask., Yukon, Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Ga., Idaho, Ill., Iowa, Ky., Maine, Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.Mex., N.Y., N.C., N.Dak., Ohio, Pa., R.I., S.Dak., Tenn., Utah, Vt., Va., Wash., W.Va., Wis., Wyo., Europe, Asia, Africa
Mnium marginatum is a widespread species that should only be confused with the closely related 4. M. lycopodioides; see discussion under that species.