Stereodon recurvatus Lindberg & Arnell
Plants yellowish to light green, blackish-brown beneath, slender, irregular to regularly 1--2(--3)- pinnate, densely to loosely (sometimes fastigiately) placed, 2--5 cm; branches unequal in length, 0.2--0.5(--0.8) cm, sometimes flagelliform, creeping and firmly affixed by rhizoids. Stems light yellow to yellowish brown, smooth, lacking hyalodermis but with weak central strand; pseudoparaphyllia frequent, possibly referable to paraphyllia, narrowly lanceolate to filamentous, rarely branched, usually with 1-seriate apex 3--6 cells in length. Leaves of stem strongly falcate-secund to circinate, 0.7--1.3 x 0.3 – 0.4mm, ovate to oblong-lanceolate, almost entire, plicate, costa indistinct, somewhat decurrent and subcordate at base, gradually narrowed to a fine acumination, margins plane or recurved near base to 2/3 or more of length, almost entire to finely serrulate, costa weak or absent, median cells 30--50 µm, sometimes longer, ca. 3 µm wide in lumen, usually thick-walled, basal cells broader and ± porose, yellowish, alar cells essentially quadrate, 4--10 in a marginal row, 10 – 15 x ca. 10 µm in lumen, thinner walled; branch leaves smaller, 0.5--1 mm, otherwise similar to stem leaves. Sexual condition. Seta reddish brown when mature, 0.7--1.5(--1.8) cm. Capsule yellowish brown to castaneous when mature, suberect to horizontal, oblong to subcylindric, 1--2 mm excluding conic operculum, 0.4--0.8 mm thick, annulus 2-seriate; cilia of endostome (1--)2--3.
Sporophytes produced summer, capsules mature late summer--fall. Obligate calcicole, usually on relatively plane surfaces of cliffs or boulders, particularly where humidity is persistent; 0-- 2000 m; Greenland; Alta., B.C., Man., N.B., Nfld., N.W.T., Nunavut, Ont., Que., Sask., Yukon; Alaska; Europe; Asia.
Hypnum recurvatum is widespread in the Northern Hemisphere, being more frequent in western, rather than eastern portions of the continents. This species has been confused with H. revolutum var. ravaudii, but the pseudoparaphyllia of that taxon are wide and foliose while those in H. recurvatum are lanceolate to filamentous. The leaves of H. recurvatum are strongly falcate to circinate, while those of H. revolutum var. ravaudii are often barely falcate and sometimes straight; also alar cell differentiation in H. recurvatum is less pronounced than in the other.