Plants prostrate, scattered or in loose mats, green to rather dark green, mostly rather sparingly branched. Leafy axes to 5 cm long, mostly tapering near the branch tips, strongly complanate with leaves imbricate, often strongly so, not at all undulate. Leaves to 2.5 mm, about 3: 1, asymmetrically oblong-ovate to ovate-lanceolate, obtuse with a minutely acuminate apex. Margins dentate to coarsely serrate at least near apex, narrowly recurved, especially immediately above the decurrency on the acroscopic side of the leaf. Decurrency distally 4–6 cells wide and several cells wide to near its base. Median cells smooth, to 20 µm wide, narrowly rhomboidal to linear, up to 12: 1, thin walled, not pitted. Alar cells, at least in part, isodiametric in a broad to auriculate cluster, often somewhat inflated. Basal juxtacostal cells rather short and not very thick-walled, rectangular, 2–5: 1, to 20 µm broad. Costa inconspicuous, short and double with the two branches unequal. Pseudoparaphyllia deltoid to orbicular, strongly overarching the bud. Cortical cells of stem broad in surface view thin-walled, rectangular, 3–8: 1; to 30 µm wide. Axillary hairs 4–5-celled, to 250 µm with a single basal brown cell, not offset from leaf insertion. Stem cross-section with an obscure central strand, with a markedly enlarged hyaloderm relative to the adjacent several layers of small and brownish somewhat pachydermous cells, and with inner cortical cells enlarged with thin and hyaline walls. Gemmae and propagules seemingly absent from California specimens.
Mostly autoicous. Perichaetia in axils of stolons, somewhat shorter than the subtending leaves. Seta to 3 cm long, erect or distally geniculate, light brown to yellow brown when mature. Capsule brown, inclined, somewhat arcuate, smooth to lightly sulcate, markedly strangulate at mouth. Urn to 3 mm, 2.5–3: 1. Exothecial cells at mid urn short rectangular, 1.5–3: 1, in regular rows, not collenchymatous, to 30 µm broad. Exothecial cells near mouth of capsule not strongly differentiated, mostly with walls somewhat thicker than those of median cells. Stomata phaneroporous, restricted to neck. Annulus present, not revoluble. Operculum short-rostrate, about 1/3 as long as urn. Exostome to 500 µm long, closely horizontally striate below, papillose above, light-brown. Endostome segments lightly papillose, narrowly or not at all perforate, about as long as exostome with basal membrane about 1/2 of total length. Cilia 2–3, nodose. Spores smooth, to 12 µm.
An unusual form of Plagiothecium denticulatum occurs at high elevations in the Klamath Mountains of Northern California. Typically this plant grows in dense wefts; its closely imbricated leaves which strongly shrink upon drying; its branch tips are usually somewhat julaceous; its leaves are very short oblong ovate, almost always entire, with a decurrency which is never auriculate. Because this suite of characters is so consistent and because the plant has a consistent ecology, I am inclined to recognize it as a new species but such a step will require more elaborate comparisons.
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