2. Ptychomitrium Fürnr., Flora. 12(2 Erg.):19. 1829.
Plants small to robust, green to yellowish green above, brown or black below, in loose or dense tufts. Stems erect or ascending, simple or branched; central strand well developed. Leaves often crisped or incurved, sometimes contorted when dry, erect-spreading to widely spreading when moist, lanceolate to linear-lanceolate or broadly lanceolate, sometimes keeled, concave above; margins erect or recurved, entire to strongly toothed above; costa single, strong, percurrent or ending below the apex; upper and median leaf cells small, rounded-quadrate to subquadrate, often bistratose, obscure, thick-walled, sometimes with sinuose walls; basal cells rectangular to elongate, thin-walled or with sinuose walls. Autoicous. Perigonia just below perichaetia. Perichaetial leaves not differentiated. Setae short to long, straight; capsules erect, symmetric, ovoid to oblong-ellipsoid, smooth; annuli none or well developed, consisting of thick-walled cells; peristome teeth 16, lanceolate to liner-lanceolate, deeply and irregularly splitting into 2–3 filiform, densely papillose segments; opercula with a short to long beak. Calyptrae mitrate, longitudinally plicate, lobed at the base, covering half to nearly the entire capsule. Spores spherical, coarsely to finely papillose or sometimes nearly smooth.
Wijk et al. (1967, 1969) listed 87 species of Ptychomitrium in the world. Crosby et al. (2000) reduced this number to 54 species, 34 of which were given their highest confidence rating (four stars). T. Cao, C. Gao and Vitt (1995) recognized eight species in China. A new species, Ptychomitrium mamillose was recently published by S.-L. Guo et al. (2000) from Sichuan province.
The distinguishing features of Ptychomitrium can be generalized as follows: 1) leaves often crisped or strongly incurved or contorted when dry; 2) calyptrae large, mitrate, covering a large portion of capsules, longitudinally plicate and lobed at the base; 3) peristome teeth haplolepideous, lanceolate to linear-lanceolate, irregularly splitting into 2–3 filiform, densely papillose segments; 4) costa single, strong, percurrent to subpercurrent; 5) the upper leaf cells rounded-quadrate to subquadrate, more or less thick-walled, smooth; and 6) the basal leaf cells elongate, thin-walled or with sinuose walls.