Plants whitish green to grayish green, sometimes yellowish, lighter in color at leaf bases. Stems 1-4(-7) cm. Leaves spreading, usually falcate-secund, 4-8 × 0.2-0.8 mm, margins usually serrulate in distal half; costa covering 1/2-2/3 of leaf base, with longitudinal striations (ridges), appearing as rows of teeth at high magnifications, on abaxial surface, especially conspicuous in distal half, in cross section with adaxial hyalocysts, median chlorocysts and abaxial hyalocysts with scattered chlorocysts in some abaxial cells. Seta 8-20 mm. Capsule 1.5-3 mm; operculum 1-2 mm. Spores 22-35 µm.
Capsules mature summer. Commonly on soil over noncalcareous boulders and cliffs, sometimes on tree trunks, stumps, and rotten logs; moderate to high (400-2900 m); Greenland; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que.; Ala., Alaska, Ariz., Colo., Maine, Mich., Minn., Mont., N.H., N.J., N.Mex., N.Y., N.C., Ohio, Pa., S.Dak., Tenn., Vt., Va., W.Va., Wis.; Europe; Asia.
Paraleucobryum longifolium is best distinguished by its 4-8 mm, whitish green to grayish green, glossy leaves that are falcate-secund, particularly at stem tips, slenderly subulate with margins usually serrulate in distal half. The costa has conspicuous fine striations or ridges, formed by small teeth when viewed at high magnifications, that are especially noticeable in the distal half even at low magnifications with a dissecting microscope. The capsules are common, 1.5-3 mm, erect, cylindric, straight, smooth, with a 1-2 mm long-rostrate operculum. This species and the next somewhat resemble a Dicranum because of the falcate-secund leaves. The species was reported from Alabama and Ohio by P. Müller and J.-P. Frahm (1987).
Paraleucobryum sauteri (Bruch & Schimper) Loeske has been considered a synonym by some bryologists (e.g., C. Barnes 1958; E. Lawton 1971). Gametophytically, it is distinguished by the costa (R. S. Williams 1913, as Dicranum sauteri Bruch & Schimper; P. Müller and J.-P. Frahm 1987) that is less than 1/3 the width of the leaf base compared to the costa that is more than 1/2 the width of the leaf base in P. longifolium, which also means more rows of laminal cells in P. sauteri than in P. longifolium. Müller and Frahm further distinguished P. sauteri by its leaf cross section which has large median cells compared to the smaller adaxial and abaxial layers of cells. Paraleucobryum longifolium, in contrast, according to them, has small median cells in comparison to the larger adaxial and abaxial layers of cells. They also found that the peristome teeth of P. longifolium are divided only to the middle and inserted at the mouth, whereas in P. sauteri the teeth are divided nearly to the base and inserted below the mouth of the capsule. Williams also used a peristome difference to distinguish the two taxa. He found that in P. sauteri the peristome teeth are punctate or nearly smooth on the exterior surface, whereas they are obliquely striate in P. longifolium. Müller and Frahm reported specimens of P. sauteri only for western North America through the Rocky Mountains from British Columbia to Colorado, South Dakota, Arizona and New Mexico. I have found the costa width in P. longifolium, however, to be quite variable in plants in eastern North America but some of the western North American plants do have a narrow costa that fits the description of P. sauteri. I also could not confirm the cross section difference between the two taxa in the few North American specimens that could be referred to P. sauteri. I have decided not to recognize P. sauteri for this flora because I believe that a detailed study of the P. longifolium-P. sauteri complex is necessary, especially in regard to the plants from the western part of the continent.