Bryol. Eur. fasc. 52–54. 1853. -- Hypnum salebrosum Web. & Mohr, Bot. Taschenb. 312. 1807.
Brachythecium laevisetum Kindb. in Mac., Bull. Torr. Bot. Club 17: 278. 1890.
Plants closely matted, green to whitish-green but with a faintly russet wash in older portions, irregularly branched, with stems prostrate and branches prostrate to ascending. Primary stems to 10 cm long with leaves similar to the closely imbricated branch leaves. Leaves to 2 mm, 2.5–3: 1, symmetric and straight, usually without a twisted acumen, lightly or not at all concave, strongly submarginally plicate, lanceolate to ovate-lanceolate with acumen only gradually differentiated. Margins entire throughout or with a few serrulations above the middle and on the acumen, somewhat recurved at base but plane above. Stem and branch leaves with a broad decurrency of quadrate to very short rectangular cells which extends to the base of the next lower leaf. Median cells nearly smooth but usually with minute verruculae, spinulose and thin-walled, to 7 µm wide, mostly 6–10: 1, not pitted. Basal juxtacostal cells short-rectangular, to 3: 1, thin-walled without pitting. Alar region strongly demarcated, with quadrate, thin-walled, enlarged but opaque cells to 20 µm wide, extending 8–10 cells toward the costa and about 4–6 cells up the margin. Marginal cells of alar region similar to the more interior alar cells. Costa ending at about leaf middle, narrow from the base but tapering to its apex, mostly spinose at apex. Juvenile leaves strongly inflexed over bud, to 150 µm long, ovate to ovate-lanceolate, 1.5–2: 1, serrulate across the acute apex. Axillary hairs persistent, to 60 µm, 2–3-celled with a single pale brown basal cell, thin-walled except at the distal end of the apical cell. Rhizoids abundantly produced on prostrate stems, elongate and planar branched, brown to reddish-brown, smooth, to 15 µm in diameter, clustered on the abaxial leaf base. Stem cross-section with central strand, a stereome 2–4 cells thick, but without an hyaloderm.
Autoicous. Perigonia bud-like, produced near the perichaetia on prostrate stems and large branches. Perichaetial leaves to 2 mm long, with costa absent or nearly so, lightly plicate, sheathing at base but reflexed to squarrose in the abruptly contracted acumen, entire throughout or with a few fine serrulations on the acumen, without alar differentiation. Seta red-brown, smooth throughout, to 20 mm long, distally geniculate. Capsule dark brown to reddish brown, inclined to horizontal, strongly arcuate, strangulate but not distorted or sulcate when dry. Urn to 1.5 mm, about 2: 1. Exothecial cells isodiametric to short rectangular, in regular rows, 20–25 µm wide, 1–3: 1, with thick lateral walls (lumen: wall ratio 3–4: 1) but with thin and oblique end walls. Stomata present at base of capsule, not on raised pustules, short and vertical pored. Operculum conic-apiculate, 1/3–1/4 as long as urn. Annulus present, of two rows of large and thin-walled cells. Exostome teeth inserted at mouth of capsule, united near base, to 550 µm long, narrowly lanceolate and obscurely shouldered, 6–8: 1, tapering to a very narrow tip, horizontally striate, yellow to yellow-brown. Endostome segments lightly papillose, about as long as exostome, with basal membrane about as long as the segments. Cilia 1–2, lightly papillose, nodose to appendiculate. Spores smooth, 15–20 µm.