Thuidium pygmaeum Schimper in P. Bruch and W. P. Schimper, Bryol. Europ. 5: 162. 1852

Stems 2-pinnate, stems and branches papillose; paraphyllia (1-)2-4(-5) cells in length, cells 1:1, strongly papillose. Stem leaves slightly incurved when dry, ovate-triangular, 0.2-0.4 mm; apex acuminate; costa 3/4 leaf length. Branch leaves erect-incurved when dry, 0.2-0.3 mm on primary branches, 0.2 mm on secondary branches; apex acute to short-acuminate; costa 1/2-3/5 leaf length, straight, not covered with cells apically. Perichaetial leaves with margins subentire proximally, serrulate distally. Seta smooth. Capsule with endostome cilia in pairs.

Capsules mature summer (Jul-Sep). Moist woods, moist rock, limestone, sandstone, quartzite; low to moderate elevations; Ont., Que.; Ala., Ark., Conn., Fla., Ga., Ill., Ind., Iowa, Ky., Mass., Mo., N.J., N.Y., N.C., Ohio, Okla., Pa., Tenn., Vt., Va., Wis.; Asia (China, Japan, Korea).

Cyrto-hypnum pygmaeum is the smallest species of the genus in the flora area; the diminutive stature coupled with papillose stems and branches is distinctive. The species is most likely confused with Heterocladium macounii (Pterigynandraceae), which also has multipapillose laminal cells and papillose stems and branches. However, H. macounii is larger and has a costa ending near or before mid leaf and is without paraphyllia; pseudoparaphyllia are present, but these have pointed apical cells, not truncate as are the paraphyllia cells of C. pygmaeum.