Stereodon curvifolius (Hedwig) Mitten
Plants medium-sized to robust, procumbent to suberect, light green to yellow-green, generally regularly pinnately branched, 4--10 cm; branches to 1.5 cm, leafy stems 0.5--3 mm wide. Stems orange brown to reddish, hyalodermis absent, central strand weakly differentiated; pseudoparaphyllia foliose. Leaves of stem ovate, falcate-secund, 1.5--2.5 × 0.7--0.8 mm, narrowing to a relatively broad acumen, curving to the insertion, slightly decurrent, margins plane, sinuose to entire near base, gradually weakly toothed near apex; costa short and double; median leaf cells 70--80 × 4--5 µm, stouter at apex and at insertion where broader and pigmented, median cells sometimes projecting abaxially at distal end of cell, especially near apex; alar cells few, hyaline and enlarged, in 1--3 rows across base, delimited by smaller ovoid to triangular cells distally. Sexual condition dioicous; inner perichaetial leaves erect, lanceolate to subulate, tapering gradually to apex, somewhat plicate. Seta 2--4.5 cm, orange-yellow to brownish when mature. Capsule reddish brown, obovoid-cylindric, 2.5--3 mm excluding conic operculum, strongly curved, plicate when dry, annulus broad, 2-seriate, cilia of endostome 2--3, nodulose.
Sporophytes produced in summer; capsules mature July--Aug. Terrestrial or most commonly on decaying logs in woodland, also rock, occasionally in peatland; 0--2000 m; N.B., Nfld., N.S., Ont., P.E.I., Que.; Ala., Ark., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.
Hypnum curvifolium is an eastern North American endemic. It might be confused with H. lindbergii from which it differs in the more extensive and often decurrent alar region of H. lindbergii and the more plumose branching in H. curvifolium. In Hypnum lindbergii, the leaves are never as circinate as those of Hypnum curvifolium. The presence of an hyalodermis in H. lindbergii and its absence in H. curvifolium further separates these species.