Plants small to robust, not polytrichoid, in loose tufts. Stems simple or rarely sparingly branched. Leaves without a differentiated sheath, membranous, strongly crisped and contorted when dry, widely spreading and plane to ± distinctly concave when moist, with a differentiated border of 1 or more rows of narrow, linear, thick-walled cells, the border usually 2-stratose, the margins toothed to mid-leaf or below with single or paired teeth (teeth and cells of leaf border papillose in A. crispum); costa narrow, abaxially toothed in distal half or rarely smooth, bearing adaxial lamellae; abaxial lamellae none; lamina 1-stratose except for margins, often obliquely transversely undulate and abaxially toothed along the undulations, or almost plane and not abaxially toothed; lamellae ± straight or tall with sinuous margins, rarely rudimentary and almost absent, entire in profile, the marginal cells in section not differentiated, smooth; basal cells of lamina short- to long-rectangular, in longitudinal rows; median cells subquadrate to transversely elongate-hexagonal, in ± regular longitudinal rows, thin- to firm-walled, with or without distinct trigones, slightly convex on both surfaces (adaxial surface bulging-mammillose in A. angustatum), smooth or with minute rounded to striate papillae. Sexual condition dioicous, polygamous, or synoicous; perigonia inconspicuous, the bracts resembling the leaves, or the bracts broadly ovate and overlapping, forming a conspicuous disc, sometimes with more than one male inflorescence in sequence per plant; perichaetial leaves usually narrower and longer than the stem leaves. Seta 1(-3) per perichaetium, straight to flexuose, twisted near the capsule when dry, smooth. Capsule short- to long-cylindric, ± straight to arcuate, terete, finely striate when dry, absent a basal constriction; stomata none; exothecium smooth, the cells elongate-rectangular, not pitted, the longitudinal walls thickened; operculum rostrate, the beak straight to arcuate; peristome teeth 32, simple, linear and crowded. Calyptra cucullate, smooth or merely hispid at tip. Spores finely papillose, appearing smooth.
Species ca. 20 (9 in the flora): North America, West Indies, Central America, South America, Europe, Asia, Africa, Australia.
Atrichum is unique in the family in having distinctly bordered, transversely undulate leaves. The peristome, however, is almost identical to that of Psilopilum, suggesting a close relationship. Inflorescence types are important differences between species, but are sometimes difficult to demonstrate. Dioicous (presumably heterothallic) species have antheridia and archegonia on separate male and female plants (A. angustatum, A. crispulum, A. crispum, A. selwynii, A. tenellum). Polygamous (heteroicous) species have both unisexual and bisexual shoots, the latter protandrous, with a male inflorescence followed by a female after an interval of vegetative growth (A. altecristatum, A. cylindricum, A. undulatum). Atrichum flavisetum is synoicous, with antheridia and archegonia within the same involucre, the perigonia hidden among the perichaetial leaves.
T. C. Frye (1937) emphasized the fractions of leaf width obscured by the lamellae, without taking into account many useful characters and important distributional differences. His treatment of distinctive North American taxa as varieties of A. undulatum has delayed recognition of unique aspects of our flora. Three species of the flora are endemic to eastern North America (A. altecristatum, A. crispum, A. cylindricum), and one to the West (A. selwynii). Atrichum crispulum is disjunctive in eastern North America and eastern Asia, whereas true A. undulatum appears to be introduced in North America.
Atrichum oerstedianum (Müller Hal.) Mitten
North American specimens named as this species are Atrichum crispulum.