Plants in loose patches, green. Stems 1--5 cm, central strand present. Stem leaves oblong to ovate-lanceolate, 2--3 × 0.8--1 mm, margins plane, incurved above, muticous, concave, rounded to cucullate, costa not projecting on dorsal side, ending well below apex; juxtacostal and marginal distal laminal cells 1-stratose; medial laminal cells quadrate, thin-walled; basal juxtacostal laminal cells quadrate, thin-walled; basal marginal laminal cells quadrate, thin-walled. Sexual condition dioicous. Seta straight, 2--3 mm, yellowish. Capsule sporadically present, exserted, yellowish, ovoid to oblong, smooth, exothecial cells thick-walled, annulus absent, operculum rostellate, peristome teeth purple, perforated, fully developed, papillose. Calyptra cucullate.

Wet acidic rocks in alpine and boreal habitats; 1000--4115 m.; Greenland; Alta, B.C., Quebec, Yukon; Calif., Colo., Mont., Wash.; Eurasia.

Grimmia mollis, discovered by Schimper in the Tirol, is a characteristic Arctic-alpine species, frequently growing in glacier streams, sometimes over stream reaches of hundreds of meters. Although the species is widespread in the Northern Hemisphere, it is nowhere common. Grimmia mollis occurs exclusively above the treeline. Because of its unique costal structure, laminal morphology and peculiar ecological niche, it cannot easily be confused with any other species. I. Hagen (1909) placed G. mollis in a monospecific genus, Hydrogrimmia. This concept was followed by M. F. V. Corley et al. (1981). Because there are no significant morphological characters to separate it from Grimmia, however, H. C. Greven (2003) follows, in agreement with H. A. Crum and L. E. Anderson (1981), the concept of the Bryologia Europaea. This dioicous species grows in separate male and female cushions and capsules are very rarely produced.