Plants pale green to yellowish green, glossy, rarely dull, in thin to dense mats. Stems to 40 × 1--4 mm, erect, sometimes prostrate, usually julaceous, rarely somewhat complanate-foliate, often flagelliform and attenuate at apices. Leaves imbricate, rarely distant, usually erect, sometimes spreading, smooth, strongly concave, rarely nearly flat, usually symmetric, 1--3 × 0.4--1.4 mm, ovate or oblong-ovate, abruptly acute or slenderly acuminate, apex often recurved, margins plane or often narrowly recurved nearly to apex, entire or rarely serrulate near apex; costa short and double, one branch often reaching leaf middle, sometimes one branch poorly developed and costa appearing single, or costa rarely lacking; leaf cells smooth, walls of basal cells pitted; median cells 60--161 × 7--17 μm; decurrent alar region triangular in outline, consisting of 1--5 vertical rows of rectangular cells, 28--70 × 12--22 μm, terminating at the base in a single cell. Specialized asexual reproduction by propagula sometimes present, 36--110 × 9--17 μm, consisting of 2--7 cells. Sexual condition dioicous, rarely fruiting. Seta light brown to red, straight or somewhat curved, 1--2.6 cm. Capsule light brown to dark red when mature, erect to inclined, straight or often arcuate, 1--2.5 × 0.3--0.8 mm, when dry contracted below mouth, smooth or often striate or wrinkled, strongly wrinkled at neck; operculum rostrate, 0.8--1 mm; cilia 1--3. Spores 9--14 μm.

Capsules mature spring--summer. Shaded soil or humus overlying boulders and cliffs, sometimes on rotten logs, stumps, and bases of trees; 40--1980 m; Greenland; B.C., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Yukon; Ala., Alaska, Ark., Calif., Colo., Conn., D.C., Ga., Idaho, Ill., Ind., Iowa, Ky., Maine, Md., Mass., Mich., Minn., Mo., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., S.C., S.Dak., Tenn., Vt., Va., Wash., W.Va., Wis.; Europe; Asia.

The julaceous plants with concave, recurved leaf apices are distinctive characters, and help distinguish P. cavifolium from the other North American species. The species is very similar to P. sylvaticum (Bridel) Bruch, Schimper & W. Gümbel of Europe; P. cavifolium, however, is usually julaceous with concave leaves that have narrow median cells, 7--17 μm wide, while P. sylvaticum is complanate-foliate with flat leaves whose median cells are broader, 12--22 μm wide. The California record is from a report by K. McGrew (1976).