Plants green to dark-green, usually with a reddish cast. Stems to 1 cm, central strand present. Stem leaves appressed to weakly spreading when dry, spreading but stiff and not keeled when moist, monomorphic, ovate to ovate-lanceolate, grooved adaxially along the costa, often cucullate near leaf apex, 0.7--1 mm, base scarcely differentiated to ovate in shape, margins weakly recurved to near apex of leaf, entire, apex broadly acute or blunt, not fragile, often ending in 1--3 celled apiculus; costa ending shortly before the apex or percurrent, seldom weakly excurrent, often weakly spurred, little widened towards apex, little tapering, sometimes rather thick and bulging adaxially, lacking an adaxial thin-walled pad of cells or this poorly developed, adaxial costal cells quadrate to short-rectangular, 4(--6) cells wide at midleaf, guide cells in 1(--2) layers; basal laminal cells differentiated medially or across the leaf, walls mostly thin, quadrate to short-rectangular, not perforated; distal laminal cells 7--10 µm wide, 1:1, nearly smooth or papillae simple or 2-fid, 2--3 per lumen, lumens quadrate or rounded-quadrate, walls somewhat thickened, weakly convex on both sides of lamina, distal leaf margins 1-stratose or occasionally 2-stratose in patches. Specialized asexual reproduction by axillary, multicellular gemmae. Seta 0.8--1 cm. Capsule 1.5--2.5 mm; peristome teeth 32, linear, twisted 1/2, 200--250 µm, delicate and commonly coming off with the operculum. Spores 10--13 µm. Distal laminal KOH reaction yellow- or red-brown.

Capsules mature spring--fall. Soil, limestone, lava, mortar, steppe, road banks, near spring, streamside, arid grassland, soil over lava, sandstone cliffs; 80--2300 m; Greenland (Disko); B.C.; Alaska, Ariz., Calif., Colo., Idaho, Mont., N.Mex., Oreg., Utah, Wash.; Mexico; Atlantic Islands (Iceland); Antarctica.

This largely arid land species has a more ovate leaf shape, margins less strongly recurved, and costa ends before the apex, which may terminate in a small conical cell or apiculus. The propagula are not always produced. Although the KOH reaction is commonly red, it may be yellow but not negative, as in D. tectorum. Didymodon luridus Sprengel (see R. H. Zander 1978b) does not occur in the range of the flora, though reported by many authors (often as D. trifarius, see discussion of R. H. Zander 1981). It differs in the triangular leaves and the smooth, more homogeneous and smaller distal laminal cells, 6--9 µm. American collections identified as this are commonly actually D. brachyphyllus, D. nicholsonii, D. tophaceus or D. vinealis. Small forms of D. nicholsonii have the leaf shape of D. brachyphyllus but the lamina is 2-stratose. Sterile Grimmia species may be confused with this species, but a small hyaline apex is commonly found on at least some leaves. Small forms of D. vinealis may be confused with D. brachyphyllus but the latter never has lanceolate leaves, and its perichaetial leaves are also short and rather deltoid. Didymodon tectorum is similar but has larger leaves, usually green in nature, broadly rectangular leaf base, and an excurrent costa. Didymodon revolutus is similar but has unicellular gemmae. The exsiccat No. Amer. Musci Perf. 404 distributed by A. J. Grout as Husnotiella torquescens, is D. brachyphyllus with operculate sporophytes demonstrating rudimentary peristomes.