Plants: large, occasionally small, in moderately dense mats, light green, becoming whitish stramineous or golden yellow in exposed habitats. Stems: to 10 cm, creeping to ascending, julaceous, irregularly or rarely regularly pinnate, branches to 16 mm, straight to flexuose, julaceous. Stem: leaves loosely appressed on proximal portions of stem, closely imbricate, ovate, broadest at 1/4–1/3(–2/5) leaf length, concave, not or slightly plicate, 2–3 × 0.6–1.2 mm; base gradually rounded, indistinctly short-decurrent; margins plane, incurved or recurved basally, entire or minutely serrulate below acumen, serrate or entire in acumen on leaves of same shoot; apex abruptly contracted, acumen filiform, long; costa to 30–60% leaf length, broad proximally, rapidly narrowing distally, sometimes shorter or 2-fid from base, terminal spine absent; alar cells subquadrate, same size as or smaller than basal cells, 13–20 µm wide, walls thick, region distinctly or indistinctly delimited, of 6–8 × 5–6 cells, opaque or moderately pellucid; laminal cells linear, 30–80(–100) × 6–7 µm; basal cells to 10–12 µm wide, walls weakly to moderately thick, region in 2 or 3 rows. Branch: leaves similar, smaller. Sexual: condition dioicous. Seta: red-brown, 1.5 cm, rough. Capsule: horizontal, red-brown, elongate, curved, 16 mm; annulus separating by fragments; operculum sharply long-conic. Spores: 18–20 µm.
Wet cliffs, rock outcrops, soil in arctic and mountain tundra, among other mosses. low to high elevations (0-4200 m). Greenland, Alta., B.C., N.W.T., Nunavut, Yukon, Alaska, Colo., Wyo., Europe, Asia.
Brachythecium cirrosum has a mainly arctic-alpine distribution, with the exception of East Asia, where it grows on trunks of evergreen trees. The species is usually unmistakable due to its strongly cirrose appearance: leaves are invariably contracted into widely spreading filiform acumina, clearly expressed in both stem and branch leaves. The alar regions reach from the margin 25–50% the distance to the costa. Distinction from 14. B. brandegeei and Cirriphyllum piliferum is discussed under these other species.