Plants in variable loose patches, dark green to black, frequently rust-coloured below. Stems 2--7 cm, central strand absent. Stem leaves lanceolate to ligulate, 1.5--3 × 0.3--0.6 mm, keeled, margin recurved proximally, incurved distally, tapering to blunt cucullate apex, muticous, costal transverse section prominent, usually terete; distal laminal cells 1-stratose with 2-stratose ridges, to completely 2-stratose; medial laminal cells rectangular, nearly straight to sinuouse or nodulose, thick-walled; basal juxtacostal laminal cells rectangular, straight to slightly sinuose walled; basal marginal laminal cells in 1--3 rows quadrate, hyaline with straight to slightly sinuose, thick transverse walls. Perichaetial leaves not enlarged. Sexual condition dioicous. Seta straight 2--6 mm. Capsule occasionally present, long-exserted, yellow-brown, obloid to cylindric, exothecial cells rectangular, thick-walled, annulus of 3--4 rows, rectangular, thick-walled, stomates present, operculum conic to rostrate, peristome present, fully developed, perforated and split distally, weakly papillose.

Damp, heavy-metal-bearing rock from the lowlands to the alpine; of conservation concern; 1100--2600 m; Greenland; Labr., Yukon; South America (Bolivia); Eurasia.

Grimmia atrata is rare in North America being known only from three widely scattered areas. It is known to geologists as one of the “copper-mosses,” i.e, it is an indicator of heavy metal-bearing rock. Because it prefers damp gneiss and mica schists, the tufts are often orange inside on account of the presence of heavy metal oxides. The placement of G. atrata has been problematic. Because of the curved distal leaves and the absence of awns, it does not have the immediate appearance of a Grimmia. As a result it has previously been placed in a separate genus, Dryptodon, intermediate between Grimmia and Racomitrium. Following T. Cao and D. H. Vitt (1986), Hastings here places it in the subg. Guembelia based on its thick, keeled leaves, long, straight seta and smooth capsules. With its recurved margin, rectangular and thick-walled sometimes sinuous basal laminal cells, prominent annulus and mitrate calyptra, it would seem most close to the group including G. longirostris and G. pilifera. However, its large size and muticous, cucullate leaves, which are often ligulate, coupled with its preference for moist habitats readily separates this species from other members of this group. In densely shaded habitats, it grows in loose patches and the areolation shows a near absence of sinuosity. On dry rock, however, the plants have extremely thick, nodulose cell walls that place the species firmly into Grimmia.