Plants 1.5-5(-8) cm. Stems red or brown. Leaves green, yellowish, or occasionally dark green, slightly contorted, usually coiled and twisted when dry, sometimes to one side of stem, narrowly elliptic, narrowly ovate-elliptic, or ovate-lanceolate, 3-6 mm, ; base short-decurrent; margins pale brown, 2-stratose, toothed to below mid leaf, sometimes to near base, teeth paired, large, sharp; apex acute or acuminate, apiculate or cuspidate, cusp sometimes toothed; costa ending well below apex or occasionally percurrent, distal abaxial surface strongly toothed, adaxial surface occasionally toothed; medial laminal cells ± isodiametric or short-elongate, (15-)17-26(-30) µm, slightly smaller towards margins, not in diagonal rows, sometimes in longitudinal rows, not or weakly collenchymatous, blue postmortal color absent; marginal cells linear, in (2-)3-4 rows. Sexual condition dioicous. Seta single. Capsule pale brown, 3-5 mm; operculum conic-mammillate, rarely short-rostrate; exostome yellowish brown. Spores 30-35 µm.

Capsules mature summer. Banks along streams, shorelines, cliff faces on moist, sandy soil, humus; low to moderate elevations; Greenland; N.B., Nfld. and Labr., N.S., Ont., P.E.I., Que.; Ala., Colo., Conn., Del., Fla., Ga., Ill., Iowa, Ky., Maine, Md., Mass., N.J., N.Y., N.C., Ohio, Pa., R.I., Tenn., Vt., Va., W. Va.; South America; Europe; Asia; Africa.

Mnium hornum is one of the most distinctive members of the genus. The long, narrow leaves and strongly toothed abaxial costae with teeth of 1(-3) cells are diagnostic.