Beih. Bot. Centralbl. 28: 207. 1911. -- Grimmia rivularis Bridel, J. f. Bot. 1800(1): 276. 1801. -- Schistidium alpicolum var. rivulare (Brid.) Limpr., Laubm. Deutschl. 1: 708. 1889.
Plants to 5 cm high, in brownish-green to somewhat reddened cushions, mostly opaque-green near growing apex. Leaves lightly imbricate when dry, erect-spreading when moist, concave and distally keeled to almost cucullate, to 3 mm long, lanceolate to ovate lanceolate, 3–4.5: 1. Median leaf cells in very regular longitudinal rows, unistratose throughout or with bistratose streaks near the apex, mostly bistratose in 2–3 cell rows at leaf margin, smooth, to 10 µm wide, isodiametric, 1–1.2: 1, with lumens regular and with distinct but small corner thickenings. Marginal cells regularly quadrate to short elliptic with rather thick walls but without transverse wall thickening. Basal juxtacostal cells short-rectangular and only moderately thick-walled with lumen: wall ratio about 3.5–4: 1, 2–3 times as long as broad, to 8 µm wide, with lateral walls straight. Cells of alar region to 8 µm wide, quadrate with rounded lumens, with lumen: wall ratio 3–4: 1, little different from more distal laminal cells. Costa filling 1/6 to 1/8 of immediate leaf base, little tapered above, mostly ending a few cells short of the leaf apex but often percurrent. Costa crosssection at leaf middle biconvex-elliptic, about 3–4 times as thick as the adjacent leaf, with 4–6 cells exposed on adaxial surface and about 1o cells exposed on abaxial surface. Leaf margin entire except for a few blunt teeth near apex, plane or recurved near the base on one or both sides. Awn not present on any leaf. Axillary hairs to 600 µm long, with more than ten rectangular, thin-walled cells, without a basal brown cell. Rhizoids dense and long in older portions of plant, red brown and monopodially branched, to 30 µm wide at base, smooth to finely verruculose. Stem crosssection round with a small and inconspicuous central strand, with stereome yellow-brown, 3–4 cells thick and well-differentiated from the somewhat pachydermous, hyaline inner cortical cells,
Autoicous with perigonia located only a few leaf axils below perichaetia. Inner perichaetial leaf pale to hyaline, ligulatenceolate, erect and about as long as the capsule. Outer perichaetial leaves loosely sheathing capsule, mostly a little longer than the adjacent vegetative leaves, never with awn. Seta to 0.5 mm long, yellow to pale brown, straight and smooth. Urn immersed, erect, symmetrical and short-cylindric to elliptic, smooth when dry, to 1.2 mm long, about 1.5: 1, not strumose. Operculum apiculate to short rostrate, to 1/5 as long as urn. Annulus absent or indistinct. Calyptra mitrate to cucullate, lacerate basally, covering only operculum. Peristome vivid red-brown to glossy brown, to 500 µm long, nearly smooth, deeply and asymmetrically cribrose but without bifid forking. Suboral exothecial cells thin-walled in 2–4 rows, transversely elongate, to 15 µm wide, about 0.7: 1. Median exothecial cells very irregular in size and shape, 15–25 µm wide, not in regular vertical rows, quadrate to rectangular or hexagonal, 1–2: 1, rather thick-walled with lumen: wall ratio about 4–5: 1. Basal exothecial cells smaller but otherwise undifferentiated. Stomata absent. Spores smooth, to 24 µm.