Stereodon hamulosus (Schimper) Lindberg
Plants small, yellowish, pinnate to irregularly branched, forming turfs of erect to suberect shoots or creeping, 2--9 cm, branches 0.4--0.8 cm, slender. Stems yellowish, with hyalodermis and weak central strand; pseudoparaphyllia scarce, foliose. Leaves of stem strongly hamate and falcate-secund, ovate-lanceolate, slightly tapered to base, 1 × 1.5 mm, margins plane, mostly entire but weakly toothed toward apex; cells narrowly vermiculate, with blunt ends, 6--7 × 60--75 µm shorter and broader toward base; alar cells rarely differentiated except at margin and near insertion, with 2--3 slightly larger, thin-walled cells. Sexual condition phyllodioicous; perichaetial leaves erect, except the outermost reflexed, plicate, with attenuate toothed apex. Seta 1--2 cm, reddish. Capsule inclined to horizontal, long-cylindric, urn 1.5--1.8 mm, operculum conic, 0.5 mm; annulus 2--3-seriate cilia of endostome 1--2, smooth.
Capsules mature July--August, infrequent. Predominantly on calcareous rocks, in crevices, also reported in tundra, mountains in southern part of its range; elevation not determined; Alta., B.C., N.W.T., Nunavut, Yukon; Greenland; Alaska, Colo.(?); Europe; Asia.
Hypnum hamulosum resembles the related species H. callichroum, H. holmenii, and H. plicatulum, especially the last two, but differs in its strongly hamate stem leaves that generally do not taper to the base and usually lack differentiated alar cells. Branch leaves are not diagnostic, and tundra forms can be impossible to name with confidence. These often possess only branch leaves as the main stem is not present. Many reports of H. hamulosum from North America are based on such specimens.