Advances in Cladistics 143. 1981 -- Grimmia occidentalis Lawt., Bull. Torr. Bot. Club 94: 461. 1967.
Grimmia alpicola var. rivularis f. acutifolia Grout, N. Am. Musci Perf. 387. 1940.
Plants to 3 cm high, in brownish-green to somewhat reddened cushions, opaque-green to brown below. Leaves lightly imbricate when dry, erect-spreading when moist, somewhat concave but not at all keeled, to 3 mm long with the awn never produced, narrowly lanceolate, 5–6: 1. Median leaf cells in very regular longitudinal rows, unistratose throughout or with a single row of bistratose cells at the margin, smooth, to 8 µm wide, isodiametric with lumens regular and with very small corner thickenings. Marginal cells not differentiated. Basal juxtacostal cells rectangular and rather thin-walled, to 2: 1, to 10 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 less than 1/10 of immediate leaf base, not tapered above, percurrent to shortly excurrent. Costa crosssection at leaf middle elliptic and biconvex, about 2.5 times as thick as the adjacent leaf, with 5–6 cells exposed on adaxial surface and 8–10 cells exposed on abaxial surface. Margin of leaf limb plane to erect throughout. Hyaline awn never produced. Axillary hairs to 500 µm long, with more than ten short rectangular, thin-walled cells, without a basal brown cell. Rhizoids red brown and monopodially branched, to 20 µm wide at base, smooth to finely verruculose. Stem crosssection round without a central strand or with that strand inconspicuous, with stereome yellow-brown and up to 3 cells deep, rather well differentiated from the leptodermous, hyaline inner cortical cells,
Autoicous with perigonia located on separate branches similar to those with perichaetia. Innermost perichaetial leaf ligulate, lightly pigmented, to 2 mm long. Outer perichaetial leaves loosely sheathing capsule, mostly only a little longer than the adjacent vegetative leaves with awn consistently absent. Seta to 0.5 mm long, yellow to pale brown, straight and smooth. Urn immersed, erect, symmetrical and oblong when operculate, smooth when dry, to 1.5 mm long, about 1.5: 1, not strumose. Operculum with apiculus about 1/4 as long as urn. Annulus absent or indistinct. Calyptra cucullate, covering only operculum Peristome vivid red, to 350 µm long, finely vertocally striate. Suboral exothecial cells thick-walled, transversely elongate, to 20 µm wide, about 0.5: 1. Median exothecial cells very irregular in size and shape, 20–30 µm wide, not in regular vertical rows, quadrate to rectangular to hexagonal, 1–2: 1, thin-walled with lumen: wall ratio more than 5: 1. Basal exothecial cells short rectangular and in regular rows but only gradually differentiated from median ones. Stomata absent. Spores smooth to finely verruculose, to 10 µm.