The following is from the California Moss eFlora. See here
Plants erect in small green to rather dark green tufts, to 5 mm high. Leaves spreading when moist but, when dry, loosely erect with apices not incurved. Leaves to 2.5 mm long, about 3: 1, ovate-lanceolate to lanceolate with obtuse to rounded but minutely apiculate apices. Median laminal cells isodiametric with rounded lumens, thick walled with strong corner thickening, to 18 µm wide, smooth or with 1–2 very low papillae on both surfaces. Basal juxtacostal cells pellucid and rectangular, 1.5–2.5:1, to 24 µm wide with lateral walls straight but somewhat unevenly thickened. Basal juxtamarginal cells isodiametric and similar to median laminal cells. Margins recurved from base to near the apex, entire or with a few serrulations at the extreme apex. Costa ending within about ten cells of apex, not situated in a keel. Costa cross-section homogeneous, at mid-leaf only 2 cells thick and up to 4 cells wide. Axillary hairs mostly rather uniform in size, to 5 cells and 100 µm long, with one basal brown cell, not offset from leaf insertion. Rhizoids abundant near base of plant but also present even on distal portions of plant, smooth, pale-brown to red-brown, to 30 µm in diameter at base, sparingly branched. Stem cross-section without a central strand, with hyaline inner corticals and with 1–2 layers of somewhat thicker-walled, red brown outer cortical cells. Gemmae scattered on adaxial leaf surface, to 25 µm in diameter, composed of 3–4 essentially isodiametric, green cells Autoicous with perigonia in leaf axils near the perichaetia or on the apices of separate short branches. Perichaetial bracts similar to the vegetative leaves. Capsule immersed to slightly emergent on a yellow to yellow brown, smooth and erect seta of about 0.5 mm in length. Urn orange-brown when fresh, to 2 mm long, 2–3: 1, erect, strongly sulcate with eight ribs, strongly strangulate. Operculum short rostrate. Annulus well-defined, falling in fragments. Exothecial cells at capsule mouth strongly reddened, to 20 µm broad, isodiametric to transversely elongate with rounded lumens in up to 4 rows. Exothecial cells at middle of urn quadrate to rectangular, 1.5–3: 1, to 30 µm broad, thick walled on ridges of capsule with lumen:wall ratio about 1: 1, thinner walled and more elongate in valleys of the sulcate capsule. Stomata present near middle of capsule wall, cryptoporous, only partially obscured by subsidiary cells. Exostome teeth strongly reflexed against capsule wall when dry, fused in pairs and appearing to be only 8 in number, light brown, to 300 µm long, finely and densely papillose. Endostome segments 8, mostly shorter than the exostome and often wanting, lightly papillose. Calyptra campanulate, mostly naked of hairs. Spores green, to 18 µm, rather coarsely papillose.
Conrad, H., & Redfearn, Jr., P. (1979). How to know the mosses and liverworts. (2nd ed.). Dubuque, Iowa: Wm. C. Brown Company Publishers.
Crum, H. (1983). Mosses of the Great Lakes forests. (3rd ed.). Ann Arbor, Michigan: University of Michigan Herbarium.
Crum, H. A., & Anderson, L. E. (1981). Mosses of eastern North America. (Vol. 2). NY, New York: Columbia University Press.
Plants: to 0.5 cm. Stem: leaves stiff, loosely erect-appressed when dry, ovate-lanceolate to lanceolate, 1.8–2.8 mm; margins revolute nearly to apex, entire; apex sharply to slenderly acute, usually apiculate, rarely obtuse, not carinate, not incurved when dry; basal laminal cells rectangular, grading to quadrate at margin, walls thin, not nodose; distal cells 14–20 µm, 1-stratose, lumina smooth, rounded, smooth or papillae 1 or 2 per cell, conic, small. : Specialized asexual reproduction occasional, by gemmae on leaves. Sexual: condition autoicous. Vaginula: with hairs absent. Seta: to 0.5 mm. Capsule: immersed to 3/4 emergent, oblong to oblong-cylindric, 1.2–1.5 mm, strongly 8-ribbed, sometimes constricted below mouth when dry; stomata immersed, mid capsule, not or to 1/2 covered by subsidiary cells, cells projecting or not, inner walls thickened or not; peristome double; prostome absent; exostome teeth 8, rarely splitting to 16, reflexed, finely and evenly papillose; endostome segments 8, not well developed, of 1 or 2 rows of cells, smooth. Calyptra: oblong, smooth, sparsely hairy or rarely naked, hairs smooth. Spores: 14–21 µm.
Trunks, lower branches, base of deciduous trees, coniferous trees, crevices of rock, open, hardwood forests, trees along roadsides. low to moderate elevations (10-1000 m). Alta., Man., Ont., Sask., Ariz., Calif., Colo., D.C., Idaho, Ill., Iowa, Kans., Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., N.H., N.J., N.Mex., N.Y., N.Dak., Ohio, Okla., Pa., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Europe, n Africa.
Orthotrichum pumilum usually can be distinguished by the strongly ribbed capsules, reflexed peristome teeth, apiculate leaves with cells larger than 14 µm, and papillae that are very small or absent. In eastern North America, this taxon is uniform and non-variable, but in the West it is much more variable. Many western specimens have capsules that are oblong-cylindric, often strangulate, over 1/2 emergent, and gradually contracted to the seta.