Mnium rostratum Schrader, Bot. Zeitung (Regensburg) 1: 79. 1802; M. longirostrum Bridel

Erect stems 1-2 cm, not dendroid; sterile stems to 6 cm. Leaves light to dark green, sometimes black with age, crisped and contorted when dry, flat or sometimes weakly undulate in older leaves when moist, broadly elliptic or ovate, 2.5-4(-7) mm; base not or weakly decurrent; margins weakly toothed distally or to near base, occasionally entire, teeth blunt, of 1 (or 2) cells; apex broadly rounded, truncate, or retuse, rarely emarginate, apiculate or rarely cuspidate, cusp smooth; costa percurrent, short-excurrent, or rarely subpercurrent; medial laminal cells short-elongate or ± isodiametric, 22-35(-45) µm, slightly smaller near margins, in indistinct longitudinal or diagonal rows, strongly collenchymatous, walls not pitted; marginal cells linear or sometimes short-linear, in 3-5 rows. Sexual condition synoicous. Seta 1-4, yellow, reddish with age, 1.5-3(-4) cm. Capsule horizontal to pendent, cylindric or oblong-cylindric, 2.5-4.5 mm, neck not distinct; operculum rostrate. Spores 22-33 µm.

Capsules mature late spring-summer. Calcareous cliffs, woods, rock, soil; low to moderate elevations; Alta., B.C., Nfld. and Labr., N.W.T., N.S., Ont., P.E.I., Que.; Alaska, Ark., Calif., Conn., Idaho, La., Mich., Miss., Mont., N.J., N.Y., N.C., Ohio, Oreg., Pa., S.C., Tenn., Tex., Vt., Wash.; Mexico (Coahuila); South America; Europe; Asia; Africa; Indian Ocean Islands; Pacific Islands; Australia.

Plagiomnium rostratum is characterized by its small size, often retuse or emarginate leaf apices, not or weakly decurrent leaves, small, blunt marginal teeth, and, in fertile collections, long-rostrate opercula. Plants are commonly observed as masses of sterile stems growing closely to the substratum.