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.