Plants loosely cespitose or forming cushions, yellowish brown, brown or blackish distally, yellowish brown to reddish brown proximally. Stems short to elongate, to 2(-3.5) cm; hyalodermis rarely present, sclerodermis present, central strand present; axillary hairs short to elongate, all cells almost always hyaline. Leaves appressed incurved to weakly spreading, often contorted or twisted about stem, occasionally catenulate when dry, spreading when moist; ligulate or broadly lanceolate to long-triangular, adaxial surface usually deeply grooved along costa, occasionally broadly concave; base weakly differentiated to broadened and somewhat sheathing, proximal margins sometimes narrowly decurrent; margins usually recurved in proximal 1/2-2/3, occasionally plane throughout, entire or occasionally weakly denticulate near apex; lamina 1-stratose; apex rounded to obtusely acute, usually mucronate, occasionally entire or apiculate; costa percurrent to shortly excurrent as a sharp mucro, occasionally ending a few cells before the apex, adaxial outgrowths absent, adaxial cells elongate, occasionally quadrate to short-rectangular, in 2-3(-5) rows; transverse section ovate to semicircular, adaxial epidermis differentiated, adaxial stereid band usually present, usually small, guide cells 2-4 in 1 layer, hydroid strand occasionally present, abaxial stereid band present, usually strong, semi-lunar in sectional shape, abaxial epidermis usually present but weakly differentiated; basal cells differentiated across leaf or reaching higher medially or occasionally marginally, rectangular, usually little wider than the distal cells, 3-5:1, walls of proximal cells thin to evenly thickened; distal medial cells quadrate, usually 1:1, 1-stratose; papillae hollow or solid, multiplex to 2-fid, 2-3 per lumen, occasionally simple or absent, cell walls thin to evenly thickened, superficially bulging on both free sides. Specialized asexual reproduction by tubers borne on proximal rhizoids or gemmae borne on axillary stalks. Sexual condition dioicous or possibly sometimes rhizautoicous. Perichaetia terminal, interior leaves sometimes strongly sheathing, little differentiated or ovate to long-lanceolate, laminal cells usually rhomboid in proximal 1/2-3/4. Seta 0.5-2.5 cm. Capsule stegocarpous, theca ovate to long-cylindric, annulus weakly differentiated to strong, of 1-3 rows of vesiculose cells, sometimes revoluble or deciduous in pieces; operculum usually long-conic; peristome teeth of 32 narrow rami, filamentous to narrowly triangular, usually strongly twisted counterclockwise. Calyptra cucullate. Spores mostly 8-12 µm. KOH laminal color reaction yellow, occasionally yellowish orange.
Species ca. 200 (6 in the flora): worldwide in temperate zones.
Barbula has been much reduced in size. K. Saito (1975) presented cogent reasons for recognizing Didymodon as distinct. Characters of importance in distinguishing Barbula are: axillary hairs almost always entirely of hyaline cells; leaf adaxially usually deeply grooved along the costa; distal laminal cell papillae rough, knobby, obscuring the lumens, and protuberant along the distal laminal margins; costa usually excurrent as a sharp mucro or an apiculus of one or more clear cells; peristome is long and twisted, and as Saito (1975) pointed out, Barbula has gemmae generally larger than those of Didymodon. The three sections are represented in the flora area: sect. Barbula, including B. unguiculata and B. orizabensis; sect. Convolutae Bruch & Schimper (Streblotrichum P. Beauvois), including B. amplexifolia, B. convoluta, and B. indica; and sect. Hydrogonium (Müller Hal.) K. Saito [Hydrogonium (Müller Hal.) A. Jaeger], including B. bolleana. For many taxa previously long-held in Barbula, see 16. Didymodon.