Plants spreading in dark green to yellow-green tufts, often prostrate with only the branch tips ascending, and with the main axes to 10 cm long. Leaves spreading to strongly spreading when moist but appressed when dry, ovate-lanceolate to lanceolate, broadest near the base, with acute to narrowly obtuse apices, to 4 mm long, about 3–4: 1. Median laminal cells to 10 µm wide, arranged in somewhat regular rows, isodiametric to slightly elongate along an oblique axis. Lumens of the median cells rounded due to the strong corner thickenings. Papillae arranged over the lumens on each surface, 1–2, often bifid. Basal marginal cells to 12 µm wide, isodiametric to short rectangular, pellucid and smooth, usually clustered in a small rounded and leptodermous auricle on at least one margin. Basal juxtacostal cells mostly smooth, rectangular, to 15 µm wide, 6: 1, pitted, reddish-brown or concolorous. Cells on adaxial surface of costa elongate and pitted in 1–2 rows. Margins markedly papillose crenate, recurved from base to near the apex. Costa ending within a few cells of apex, lying in a sharply defined keel. Costa cross-section nearly homogeneous but with a single layer of abaxial cells smaller than the more adaxial ones. Costa mostly 4 cells thick, and with more than 10 cells along the abaxial perimeter. Median axillary hairs with one basal brown cell, not offset from leaf insertion, to 12 cells and 500 µm long with the laterally inserted ones much shorter than the lateral ones. Rhizoids very sparsely produced at base of plant, to 30 µm in diameter at insertion, smooth, brown to pale-brown, sparingly branched. Stem cross-section without a central strand, mostly with leptodermous and hyaline inner corticals and with 1–3 layers of strongly pachydermous, red brown outer cortical cells.
Autoicous with perigonia in leaf axils near the perichaetia. Perichaetial bracts not differentiated from vegetative leaves. Capsule immersed to somewhat emergent on a yellow to yellow brown, smooth and erect seta which seldom exceeds 1.5 mm in length. Urn pale-brown, to 2 mm long, 1.5–2: 1, erect, strongly or lightly sulcate with eight well defined ribs, lightly or not at all strangulate. Operculum conic-apiculate to short rostrate. Annulus well-defined, falling in fragments. Exothecial cells in cross-section single-layered. Exothecial cells at capsule mouth often strongly reddened, to 35 µm wide, transversely elongate with rounded lumens in up to 6 rows. Exothecial cells at middle of urn rather thick walled with lumen: wall ratio 4–6: 1, short rectangular, 2–4: 1, to 25 µm broad, strongly differentiated from ridge to valley of the sulcate capsule. Stomata phaneroporous, scattered on proximal 1/2 of urn. Exostome teeth erect to spreading when dry, seldom or never fused in pairs, pale to light yellow, to 300 µm long, lightly or not at all papillose. Endostome segments consistently absent. Calyptra campanulate, lightly covered with papillose hairs. Spores green, to 20 µm, rather coarsely papillose.
|Literature||Bourell 1981; Bradshaw 1926; Harpel 1980a; Harthill et al. 1979; Holmberg 1969; Kellman 2003; Kingman 1912; Koch 1950a, 1951e; Koch and Ikenberry 1954; Lawton 1971; Lesquereux 1868; McCleary 1972; Millspaugh and Nuttall 1923; Mishler 1978; Sayre 1940; Shevock and Toren 2001; Showers 1982; Sigal 1975; Smith 1970; Spjut 1971; Steere 1954; Steere et al. 1954; Sullivant 1856; Thomson and Ketchledge 1958; Toren 1977; Watson 1880; Whittemore and Sommers 1999; Yurky 1990, 1995. As Orthotrichum lyellii var. howei Cardot and Thériot 1900.|
|Illustrations||Malcolm et al. 2009 p. 209; Lawton 1971; Sharp et al. 1994; Smith 1978; Vitt 1973.|
|Bioregions||CaR, CW, DMoj, NW, SN, SW.|
|Vouchers||Contra Costa Co.: Mt Diablo, Brewer 844 (UC); Kern Co.: Cedar Creek along Highway 155, west of Greenhorn Summit, Greenhorn Mountains, Sequoia National Forest, Shevock 10790; Lake Co.: Highway 53 about 1 mile south of Highway 20, Norris 47692; San Diego Co.: Heise County Park south of Julian, Norris 50739; Santa Cruz Co.: Laurel Road at Highway 17, Kellman 2712 (CAS); Siskiyou Co.: Duck Lake Trail near Parrott's Mill Road, Klamath National Forest, Norris 22805.|