Plants in loose tufts, dark to light green, dull. Stems 3--8 cm, densely tomentose below with reddish brown rhizoids. Leaves falcate-secund, flexuose, contorted when dry, slightly undulate, (5--)6.5--9(--10) mm ´ 1--1.5 mm, concave below, keeled above, from a lanceolate base to a long-acuminate apex; margins plane, strongly serrate above; laminae 1-stratose, rarely with 2-stratose regions on margins or near costa; costa percurrent to slightly excurrent, 1/6--1/5 the width of the leaves at base, papillose to spinose above on numerous, low abaxial ridges, with a row of guide cells, two thick, well-developed stereid bands above and below extending to apex, adaxial epidermal layer of cells not differentiated, the abaxial layer differentiated; cell walls between lamina cells not bulging or weakly bulging; leaf cells prorate on abaxial surface, sometimes papillose adaxially; alar cells 2-stratose, rarely 3-stratose in parts, differentiated, not extending to costa; proximal laminal cells rectangular, pitted, (22--)40--64(--126) ´ (2--)7--8(--10) µm; distal laminal cells short-rectangular to rectangular, not pitted, ± in longitudinal rows, marginal cells usually more elongate, some distal cells with long tooth-like projections, scattered on abaxial surface, (7--)12--18(--42) ´ (5--)8--12(--16) µm. Sexual condition pseudomonoicous; dwarf males on stem rhizoids of female plants; interior perichaetial leaves abruptly long-acuminate, convolute-sheathing. Seta 2--3.5 cm, often aggregate 2--5 per perichaetium, sometimes solitary, yellow, brown or reddish yellow. Capsule 2--3 mm, arcuate, inclined to horizontal, furrowed when dry, yellow to light brown; operculum 1.5--3 mm. Spores 9--20 µm.

Capsules mature in spring. Humus, soil, or soil over rock, rarely rotten logs or stumps, usually in mesic to dry coniferous woods, sometimes swamps and bogs; 60--1200 m; Alta. (Peterson 1979), Man., N.B., Nfld., N.S., Ont., P.E.I., Que., Sask.; Maine, Mass., Mich., Minn., N.H., N.J., N.Y., w N.C., Ohio, Pa., e Tenn., W.Va., Wis., ne Wyo.

This North American endemic species was previously mistaken for D. drummondii J. K. A. Müller, a predominately Eurasian species which was unknown on this continent until it was discovered just recently on the Aleutians Islands. W. Peterson (1977) clarified the taxonomic position of D. ontariense, while P. Isoviita (1977) amplified on the complex nomenclatural details of why it must be recognized as a species distinct from D. drummondii. The main distinguishing features of D. ontariense are the dark to light green, dull, falcate-secund, flexuose tipped, slightly undulate leaves, contorted when dry, keeled above from a lanceolate base to a long-acumination, the leaf apices papillose to spinose above on abaxial surface, the laminae 1-stratose except for occasional 2-stratose regions on margins, the leaf margins plane, strongly serrate above, and the usually multiple sporophytes, 2--5 per perichaetium.

This species is sometimes confused with an environmental form of D. undulatum that has nearly percurrent costae and ± acute apices. The aggregate setae of D. ontariense will immediately distinguish it from that form which has solitary setae. The dull leaves that are contorted when dry, and the leaf apices that are long-acuminate, abundantly spinose on the abaxial surface, are other superficial features to distinguish it from D. undulatum, which has shiny, usually broadly acute, smooth or serrulate leaf apices. Also, on occasion, D. ontariense can be confused with large forms of D. condensatum when sterile. The thin-walled, short-rectangular to quadrate distal-median cells in more or less regular rows of the former will distinguish it from the latter, which has irregularly shaped cells that are thick-walled and usually not in well defined rows. When fruiting, the aggregate setae on some of the plants of D. ontariense will immediately distinguish it from D. condensatum, which has solitary setae.