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

Plants in extremely hoary, compact glaucous cushions, brown inside. Stems 2--3 cm, central strand absent. Stem leaves loosely appressed to slightly contorted when dry, erectopatent when moist, lanceolate, 0.5--1 × 0.3--0.4 mm, keeled, narrowly recurved on both sides, awns 2--4 mm, smooth, flattened below, decurrent, costa weak, projecting on dorsal side; distal laminal cells 1-stratose, margins 1-stratose; medial laminal cells short-rectangular, slightly sinuouse, thick-walled; basal juxacostal laminal cells rectangular, straight, thick-walled; basal marginal laminal cells quadrate with thickened transverse walls. Gemmae absent. Sexual condition dioicous. Seta flexuose, 2 mm. Capsule occasionally present, exserted, brownish, ovoid, wide-mouthed, exothecial cells thin-walled, annulus present, operculum conical, peristome teeth yellow, split and perforated distally, slightly papillose.

Vertical faces of shaded, calcareous cliffs; of conservation concern; 500--700 m; Mont.

Grimmia brittoniae is an endemic of Montana. It was described by R. S. Williams based on a set of specimens that he collected near Columbia Falls. In 1960, W. B. Schofield collected G. brittoniae with sporophytes in the Cabinet Mts. of extreme western Montana. In 1997, T. Spribille found G. brittoniae in the Bitterroot Mts. near the border with Idaho, about 20 km south of the locality discovered by Schofield. Here, the species was relatively common, and sporophytes, while rare, were present. Grimmia brittoniae grows in warm, dry but climatically moist valley-bottom or piedmont forests dominated by Douglas Fir. It is distinct and easily recognized in the field. The often extensive cushions found on rocky underhangs are a characteristic blue-green color when moist, cracking into polygonal patterns when dry. The extremely long awns at once separate it from all other species of the genus. Grimmia brittoniae can only be confused with small forms of G. funalis, which also may grow in compact hoary cushions that are glaucous green in the distal part and brown inside. J. Muñoz (2000) synonymized G. brittoniae with this species. H. C. Greven and T. Spribille (1999), however, had already made it clear that in G. funalis the leaves are usually spirally curved, the distal areolation is 2-stratose, the proximal leaves are blackish with short awns, the margins are plane or recurved on one side, and the basal cells are linear, thick-walled and sinuose. In addition, G. funalis is characterized by male plants growing in separate cushions, with muticous to very short-awned leaves. Male plants of G. brittoniae grow intermingled with female plants and can hardly be separated from them. Grimmia brittoniae is more closely related to G. orbicularis. The leaves of both species are similar, however, the awns of the latter species are much shorter, it is autoicous, usually with capsules on arcuate setae, the peristome teeth are broad, cribrose, irregularly cleft at the apex, and the operculum is mammillate. Although in the protologue, R. S. Williams described G. brittoniae as having concave leaves with plane, sometimes 2-stratose margins, the leaves are not concave but keeled distally, the margins are not plane but narrowly recurved and are only very rarely 2-stratose. The recurved margins, however, are only present in leaves still attached to the stems; once detached and pressed under a cover glass, the margins appear plane.