Hypnum collinum Schleicher ex Müller Hal., Syn. Musc. Frond. 2: 429. 1851; Brachythecium collinum (Schleicher ex Müller Hal.) Schimper; B. collinum var. idahense (Renauld & Cardot) Grout; B. idahense Renauld & Cardot; H. hillebrandii Lesquereux

Plants small, in loose to dense tufts, green usually only when very young, pale yellow or brownish yellow with age. Stems 2-4 cm, creeping to ascending, in dense tufts erect, terete-foliate, julaceous or subjulaceous, irregularly pinnate, branches to 4 mm, terete-foliate. Stem leaves appressed proximally, closely imbricate, straight to falcate, ovate to broadly ovate-lanceolate, not to rarely moderately plicate, 0.5-1(-1.5) × 0.2-0.5(-0.7) mm; margins plane or recurved proximally, coarsely serrate almost to base; apex abruptly or rarely gradually short-acuminate, acumen somewhat spreading to reflexed, often homomallous to falcate-secund; costa to 40-70% leaf length, occasionally almost absent, terminal spine absent; alar cells subquadrate, 8-20 × 9-11 µm, region small, of 10 × 4-6 cells, distinctly delimited; laminal cells elongate to linear, 25-60 × 4-7µm; basal cells almost undifferentiated. Seta reddish orange, 0.6-1.4 cm, slightly roughened, sometimes almost smooth except basal 1/4. Capsule slightly inclined to horizontal, reddish orange, ovate to subcylindric, 1-1.8 mm; endostome basal membrane 1/3 endostome length or longer, cilia as long as segments. Spores 9-12µm.

Soil, rock, xeric to mesic sunny to moderately shaded habitats; low to high elevations (0-3700 m); Greenland; Alta., B.C., Sask., Yukon; Ariz., Calif., Colo., Idaho, Kans., Mont., Nebr., Nev., N.Mex., N.Dak., Oreg., S.Dak., Tex., Utah, Wash., Wyo.; Mexico (Baja California, Chihuahua); Europe; Asia; n Africa.

Although stems of Brachytheciastrum collinum are usually julaceous, the leaves are sometimes well spaced, making some populations difficult to distinguish from Brachythecium bolanderi. In this case, the rather extensive alar group and the autoicous sexual condition of B. collinum are helpful, as well as the more abruptly short-acuminate leaves. Some authors distinguish B. collinum and B. fendleri by the non-plicate leaves of the former. However, some collections, for example New Mexico, Santa Fe County, Buck 39717, have plicate leaves, while leaves of B. fendleri are usually smooth.