Authors: Roxanne I. Hastings & Dr. Henk C. Greven

Plants in hoary, usually hemispherical cushions, greyish green. Stem 2--5 cm, central strand present. Stem leaves appressed and twisted when dry, erect when moist, broadly lanceolate, abruptly contracted into hair-point, 2--2.5 × 0.4--0.6 mm, keeled, margins recurved in the middle of the leaf on one or both sides, awns short to long, smooth to denticulate, costa weak below, projecting on dorsal side; distal laminal cells 1-stratose; medial laminal cells subquadrate, sinuose, thick-walled; basal juxtacostal laminal cells elongate, nodulose, thick-walled; basal marginal laminal cells rectangular with thickened transverse walls. Gemmae absent. Sexual condition autoicous. Seta arcuate, 2--3 mm. Capsule usually present, exserted, bent down into the cushions by the arcuate setae, yellowish-brown to chestnut-brown, ovoid, smooth, when dry and empty wide-mouthed, wrinkled and ribbed, exothecial cells thin-walled, annulus present, operculum mammillate, peristome teeth orange, broad, cribrose and irregularly cleft at apex. Calyptra cucullate.

Dry basic rock like limestone, basalt, and mortar; 200--2000 m; Ariz., Calif., Mont.; Mexico, Central America (Guatemala); South America (Argentina, Chile); Europe; north Africa; Pacific Islands (New Zealand); Australia.

Grimmia orbicularis is a thermophilous species with a preference for sunny, basic substrates. In North America it is known only from scattered localities in three western states. Grimmia orbicularis may be confused with G. pulvinata as they form comparable hemispherical cushions and usually grow in the same habitat. However, they differ markedly both in gametophytic and sporophytic characters. G. orbicularis has leaves with elongate basal juxtacostal cells with thick and nodulose lateral walls, and 1-stratose margins while Grimmia pulvinata has leaves with short-rectangular thin-walled basal juxtacostal cells and 2-stratose margins. G. orbicularis has globose capsules with broad, orange, cribrose and cleft peristome teeth and mammillate operculum while Grimmia pulvinata has obloid capsules with fully developed dark red peristome teeth and rostrate operculum.