1. Rhytidiadelphus triquetrus (Hedwig) Warnstorf, Krypt.-Fl. Brandenburg. 2: 920. 1906.
Hypnum triquetrum Hedwig, Sp. Musc. Frond., 256. 1801; Rhytidiadelphus triquetrus var. beringianus (Cardot & Thériot) Grout
Plants to 20 cm, coarse. Stems 3-8 mm wide across leafy stem, irregularly to unevenly pinnate, branches to 2.5 cm.
Soil, humus, less often on logs and rock, forests from dry-mesic oak/hickory to wet Thuja swamps, most common in northern coniferous forests; low to high elevations (0-2000 m); Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Ariz., Ark., Calif., Colo., Conn., Idaho, Ill., Ind., Iowa, Maine, Md., Mass., Mich., Minn., Mo., Mont., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., R.I., S.Dak., Tenn., Vt., Va., Wash., W.Va., Wis., Wyo.; Eurasia; Pacific Islands (New Zealand).
Stem leaves wide-spreading, heteromallous, crowded at stem apices, ovate-triangular, plicate, rugose, 3.2-4.9 × 1.6-2.5 mm; base cordate-clasping, broadly rounded to insertion; apex gradually narrowed, acumen broad, flat (occasionally rugose); costa 1/2-2/3 leaf length; alar cells slightly differentiated; medial laminal cells 30-55 × 5- 9 µm, coarsely prorate. Branch leaves ovate to lanceolate, 1.8-3.1 × 0.7-1.6 mm. Capsule oblong-cylindric, 1.8-3.5 mm.
A specimen of Rhytidiadelphus triquetrus from Florida in the Torrey Herbarium (NY) is correctly identified, but the locality is very unlikely. The branches of R. triquetrus are often arcuate-decurved.