15. Hydrogonium (C. Müll.) Jaeg., Ber. Thätigk. St. Gallischen Naturwiss. Ges. 1877–78:405. 1880.
Plants mostly medium-sized to large, sometimes small, usually grayish green, in dense tufts. Stems erect or inclined, simple or branched; central strand present. Leaves appressed, rarely contorted when dry, erect-spreading when moist, triangular- to ovate-lanceolate or ligulate, acute or blunt at the apex, plane or weakly keeled in the upper 1/3 leaf length; margins mostly plane, occasionally revolute, entire, rarely serrulate near the apex; costa stout, percurrent to shortly excurrent, with 2 stereid bands in cross section, cells of adaxial surface of costa quadrate to short-rectangular, papillose or smooth, thin-walled; upper leaf cells loose, usually quadrate to hexagonal, thin-walled, mostly smooth, rarely mammillose or papillose, well defined in surface view; basal cells rectangular, smooth, hyaline. Gemmae usually present, in various shapes. Dioicous. Perichaetial leaves smaller than stem leaves, not sheathing at the base. Setae slender, straight, yellowish; capsules erect, cylindrical; annuli differentiated; peristome teeth linear, usually erect, rarely twisted counterclockwise; opercula long conic-rostrate. Calyptrae cucullate, smooth. Spores spherical, brownish, finely papillose.
Most species of Hydrogonium occur in tropical and subtropical regions of the world, most commonly on calcareous rocks in moist habitats. Twelve species are found in China.