Stems with hyalodermis present or absent. Leaves radially and spirally arranged, lanceolate, oblong-lanceolate, linear-lanceolate, or oblong-elliptic, symmetric, weakly concave to keeled; base broad, decurrent, tapered, scarcely tapered, or slightly rounded to insertion; margins often irregularly serrulate at apex; laminal cell abaxial and adaxial surfaces plane to somewhat domed, usually 1-papillose, papillae prominent, walls collenchymatous or not. Specialized asexual reproduction by leaflike propagula or absent. Sexual condition dioicous; perigonia as terminal discoid splash platforms, or budlike in series down stem; perichaetial paraphyses absent or of 9–12 cells. Seta usually single, red basally, yellow distally. Spores 8–20 µm, smooth or slightly papillose.
North America, Mexico, West Indies, n, w South America, Europe, Asia, Africa, Pacific Islands, Australia.
Species 4 (4 in the flora).
The outer perigonial leaves of Aulacomnium have flaring apices; the inner leaves are concave with narrowed limbs and broad or conspicuously broad bases. The outer perichaetial leaves sometimes have flaring apices; the inner leaves are narrowly lanceolate and plicate with erect, long-acuminate apices and margins sometimes serrulate at the apex. The basal laminal cells of cauline leaves of this genus are large and smooth.
|1||Plants 1-2 cm; propagula present, in tight globose cluster; leaf bases slightly rounded to insertion; rhizoids between leaves near base of plants.||Aulacomnium androgynum|
|1||Plants 3-10 cm; propagula absent or present, bullet-shaped, in elongate series; leaf bases not rounded to insertion; rhizoids between leaves throughout plants or completely hidden by imbricate leaves||> 2|
|2||Leaves oblong to elliptic; apices rounded-cucullate; laminal cell walls collenchymatous.||Aulacomnium turgidum|
|2||Leaves oblong-lanceolate or broadly to narrowly lanceolate; apices acute to long-attenuate, or narrowed, rounded, somewhat cucullate; laminal cell walls collenchymatous or not||> 3|
|3||Leaves lanceolate to oblong-lanceolate, widest mid leaf, concave, strongly imbricate, completely hiding rhizoids; margins revolute (1.5-2 times) nearly to apex; laminal cell walls strongly collenchymatous.||Aulacomnium acuminatum|
|3||Leaves broadly to narrowly lanceolate, widest at base, often weakly concave, leaves distant, rhizoids conspicuous when dry or moist, or leaves imbricate and rhizoids visible among leaves at base of plants; margins revolute (1 time, sometimes more) only in proximal 2/3 of leaves; laminal cell walls collenchymatous or not.||Aulacomnium palustre|