This distinctive species can form extensive patches, usually dark green, but sometimes yellower when well-lit. The erect stems reach 7 cm, with leaves to 1 cm in length with toothed margins.  The leaves are rippled tranversely with undulations running across the leaves.  Using a hand lens, several plates of tissue can be seen as dark lines running longitudinally up the leaf over the nerve.

Cylindrical capsules are frequent, 3 to 4 mm long with a long weak beak of similar length, on a 2 to 4 cm long reddish seta.

Usually found in shaded, well-drained woodlands on all but the most extreme soils. Can also be found on heaths, grasslands or rocky ledges.

Very common and widespread in Britain.

Frequent in Leicestershire and Rutland.

https://www.britishbryologicalsociety.org.uk/learning/species-finder/atrichum-undulatum/