Plagiothecium muellerianum Schimper, Syn. Musc. Eur., 584. 1860; Isopterygium muellerianum (Schimper) A. Jaeger

Plants in thin mats, yellowish. Stems 2 cm, 0.8-2 mm wide, complanate-foliate; epidermal cells large, walls thin, cortical cells in 1-several layers, small, walls thick. Leaves somewhat rigid, erect-spreading, crowded, imbricate, ovate- to oblong-lanceolate, weakly concave, 0.5-1.5 × 0.3-0.5 mm; margins plane, entire or minutely serrulate; apex acuminate, often abruptly so; alar cells not differentiated or sometimes short-rectangular, 8-24 × 4-7 µm; medial laminal cells 55-94 × 3-6 µm. Specialized asexual reproduction sometimes present as propagula, 66-96 × 6-12 µm, of 2-6 cells. Sexual condition dioicous. Seta brown to reddish, 0.6-1.2 cm. Capsule erect or rarely ± cernuous, yellowish to orange-brown, ellipsoid to ovoid, straight, 0.5-1.5 mm; operculum conic to obliquely rostrate, 0.4-0.8 mm. Spores 8-12 µm.

Capsules mature fall (Sep). Acidic cliffs, rock outcrops, boulders in woods; moderate elevations (300-1200 m); B.C., N.B., Nfld. and Labr., N.W.T., N.S., Ont., Que., Yukon; Alaska, Ark., Conn., Ill., Ky., Maine, Md., Mich., Minn., Mo., N.H., N.J., N.Y., N.C., Ohio, Pa., S.C., Tenn., Utah, Vt., Va., Wis.; Mexico (Distrito Federal); Europe; Asia.

Isopterygiopsis muelleriana is easily distinguished from the other two species in the genus by stems that are complanate-foliate, pectinate, appearing concave on the adaxial surface because of the upturned leaves, and by epidermal stem cells that are very large and thin-walled. Inclusion here of the Utah record is based on S. Flowers (1973).