4. Helicodontium (Mitt.) Jaeg. 旋齿藓属 xuan-chi xian shu
Ber. Thatigk. St. Gallischen Naturwiss. Ges. 1876–77: 224. 1878. Hypnum sect. Helicodontium Mitt., J. Linn. Soc., Bot. 12: 545. 1869.
Plants slender, delicate, dark green, not glossy, in creeping mats. Stems prostrate, irregularly branched; branches short, ascending or arcuate, densely foliate; rhizoids sparse; central strand not differentiated. Leaves ovate- lanceolate, acuminate above; leaf margins plane, entire below, toothed above; costa slender, ending in the middle or above leaf middle; leaf cells oblong-elliptic or oblong-hexagonal; alar cells quadrate, in several rows. Autoicous. Inner perichaetial leaves sheathing at base. Setae short, 5–7 mm long, slender, reddish brown, twisted when dry, smooth or papillose; capsules ovoid or shortly cylindrical, constricted below the mouth; annuli persistent; opercula shortly rostrate; peristome double; exostome teeth lanceolate, yellowish brown, keeled, with zig-zag median line; endostome segments linear, as long as exostome teeth, smooth or finely papillose; basal membrane low. Calyptrae cucullate, smooth. Spores spherical, smooth.