The genus Pleuridium (Ditrichaceae) is revised on a worldwide basis. Pleuridium species feature immersed, cleistocarpous, basally stomatose capsules, cucullate calyptrae, and a paroicous or autoicous sexual condition. Mostly ephemerals or annuals, the species are widespread in the temperate regions and at higher elevations in the tropics. Twenty-one species are classified in two sections: Section Pleuridium includes P. ravenelii, P. acuminatum, P. subulatum, P. papillosum, P. longirostre, P. arnoldii, P. curvisetum, P. costesii, and P. pappeanum (9 species) while section Sclerastomum includes P. nervosum, the designated type species of the section, P. robinsonii, P. sullivantii, P. mexicanum, P. laxirete, P. subnervosum, P. andinum, P. venezuelanum, P. valetonii, P. denticulatum, P. sinensis, and P. holdridgei (12 species). Two widespread, polymorphic, and morphologically similar species, P. subulatum and P. acuminatum, have caused much confusion since their inception. Correlations of sexual condition and leaf lamina thickness with chromosome numbers show that the two species are distinct.

Key words: embryophytes, monographic study, mosses, taxonomy