Orthotrichum pumilum Sw.
Family: Orthotrichaceae
orthotrichum moss
[Orthotrichum brachytrichum ,  more]
Orthotrichum pumilum image
Charles DeLavoi  
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.