Plants: small to fairly large, dark green. Stems: 2–5 cm. Leaves: sparse below, more crowded above, 4–9 × 0.8–1.5 mm, lingulate to lanceolate, undulate, keeled distally, concave to flat proximally, with oblique rows of abaxial teeth on undulations, the apex acute; costa percurrent, with teeth on abaxial surface in distal half; lamellae 2–6, 2–4(–7) cells high; median leaf cells 20–32 µm wide, irregularly angled to hexagonal, sometimes rounded, often transversely elongate, convex on both surfaces, smooth or with minute, verrucose or striate papillae on abaxial surface, walls firm, strongly collenchymatous with trigones. Sexual: condition polygamous, some shoots bisexual, others apparently male or female; perigonial bracts broad, forming antheridial buds, often several buds per plant. Seta: 1(–3) per perichaetium, 1–3 cm, reddish brown. Capsule: 2–4(–8) × 0.5–1 mm, curved to distinctly arcuate, almost horizontal; operculum 2–3 mm. Spores: (12–)16–28 µm. Phenology: Capsules mature spring–summer (Apr- Jul).
Soil, dry weedy habitats, especially roadside ditches. usually low elevations. Introduced; B.C., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Maine, Europe, w, c Asia, n Africa.
No Atrichum species occurring in North America has been as widely misunderstood as A. undulatum. Evidently introduced from Europe, its weedy habitat and strongly arcuate, almost horizontal capsules are the surest means of recognition. Variety minus, a fixture of European manuals, is a stunted form that is highly variable and possibly a hybrid. Its occurrence in North America has not been demonstrated. As used by American authors (e.g., O. E. Jennings 1951), it probably refers to A. altecristatum.