Plants in loose tufts, olive green to black. Stems 0.5--0.9 cm. Stem leaves oblong-ovate to oblong-lanceolate, 1.6--2 × 0.3--0.6 mm, concave, awn 0.3--0.6 mm; distal laminal cells 2-stratose, marginal cells 2-stratose; medial laminal cells rounded, straight, thick-walled; basal juxtacostal laminal cells quadrate to long-rectangular, straight, thin-walled; basal marginal laminal cells quadrate to short-rectangular, straight, thin-walled. Sexual condition dioicous. Seta sigmoid, 0.3--0.5 mm. Capsule usually present, exothecial cells thin-walled, annulus of 2--3 rows, rectangular, thick-walled, operculum obliquely rostrate, peristome present, rudimentary, perforated, papillose.

Basalts, granites, schists and limestone; 500--2100 m; B.C., N.W.T.; Ariz., Calif., Col., Nev., N.Mex., Utah; Eurasia.

In North America, Grimmia poecilostoma is known from only scattered localities in the American southwest and an extremely disjunct site in Canada near the Keele River of the Northwest Territories. It is found on a broad range of both basic and acidic rock types. The subgeneric placement of Grimmia poecilostoma is problematic. Gametophytically the species is inseperable from Grimmia tergestina, a member of the subg. Litoneuron. Indeed, based on areolation and leaf shape, L. Loeske (1913) placed it as a subspecies of Grimmia tergestina. This close similarity may account for reports by J. Muñoz and F. Pando (2000) and D. H. Norris and J. R. Shevok (2004) of Grimmia tergestina from North America. Cited specimens that we have seen are all G. poecilostoma and so we reject G. tergestina being in North America. The species can be separated based on their sporophytic characteristics and it is for these reasons that Hastings places G. poecilostoma in the subg. Gasterogrimmia. It has a short, curved to sigmoid seta that is eccentrically attached to the capsule; an immersed ventricose capsule with a small, mitrate calyptrae; and 3--4 large stomates -- all characteristic of subg. Gasterogrimmia. The two species never grow together so there is also some ecological separation. Despite its dioicous sexuality, G. poecilostoma is usually fertile; its rudimentary peristome and large annulus are thus readily evident. Its 2-stratose laminal stratification separates it from specimens of G. plagiopodia that may have broken peristome teeth.