Plants without deciduous distal leaves. Leaf apex acute; costa subpercurrent; sheath clear or yellowish; limb green, pellucid; limb-sheath transition abrupt, not sharply angled; limb margins strongly dentate in distal half, with fewer and smaller teeth below, and a few small sharp teeth at limb-sheath transition; limb lamina cells (5-)7-16(-21) × (6-)7-12(-14) µm, with low rounded or flattened mamillae on the adaxial surface, abaxial surface smooth; abaxial surfaces of sheath lamina cells smooth or with 1-6 papillae over lumen; cells at leaf insertion not fragile and hyaline; abaxial surface of limb costa smooth (rarely papillose near leaf apex), adaxial cells with rounded or somewhat flattened mamillae. Sexual condition monoicous; perichaetial leaves narrower than vegetative leaves. Calyptra with a median longitudinal split, sometimes persistent on the neck of the capsule or seta. Capsule plicate or puckered when dry; exothecial cells with non-sinuose walls; stomata on urn; endostome cilia with numerous spine-like or branched-stellate appendiculations on the interior surfaces.
Subspecies 2 (2 in the flora): North America, Eurasia, n Africa, Atlantic Islands (Iceland), Pacific Islands (Hawaii).
Timmia megapolitana is a distinct species within the genus, separated by its monoicous sexuality, distinct but pale leaf sheaths, and elaborately appendiculate endostome cilia.
Cells of mid-limb lamina (8-)9-12(-14) µm wide (mean 10.5 µm); cells of distal part of leaf sheath with 1-6 papillae on the abaxial surface. Calyptra often remaining attached to setae at base of capsule.
Almost always with sporo-phytes. Most often in deciduous forests, on humus in moist, shady calcareous sites, northwards in forested localities, especially along major rivers, and as an adventive in disturbed sites such as lawns, golf courses or cemeteries; low to moderate elevations (0-500 m); Alta., B.C., Man., N.W.T., N.S., Ont., Que., Sask., Yukon; Alaska, Ark., Conn., Ill., Ind., Iowa, Ky., Mass., Mich., Minn., Mo., Mont., Nebr., N.J., N.Y., Pa., S.Dak., Tenn., Vt., Va., Wis.; Eurasia (China, Finland, Germany [extinct], Netherlands, Russia).
Subspecies megapolitana is the only taxon in the genus that is not an arctic or montane moss; it occurs primarily in temperate regions, and sporadic in the boreal region. It is also the only taxon occurring in man-made habitats. The common name reflects the resemblance of the erect, persistent calyptra to the feather headdresses used by some North American Indians.