Plants with deciduous longer distal leaves with a less well–developed sheath. Leaf apex acute to slightly acuminate; costa percurrent or subpercurrent; sheath clear in the distal leaves, clear or dark reddish-black in the proximal leaves; limb green, non-pellucid; limb–sheath transition gradual; limb margins dentate in distal 1/3--1/2, crenulate or slightly toothed proximally, limb lamina cells (8--)11--18(--22) X (9--)10–-19 µm, mamillose and with 2--4 papillae on the adaxial surface, with 2--4 papillae and generally also mamillose on the abaxial surface; cells of the sheath lamina with (0--)1--8(--10) large round, often verrucose papillae over the abaxial surface of the lumens; cells at leaf insertion abruptly differentiated into 1 to a few rows of fragile, hyaline cells; limb costa with the abaxial cells uni- or pleuripapillose, and the adaxial cells mamillose and pleuripapillose. Dioicous. Perichaetial leaves longer and more evenly tapering to the apex than stem leaves, with the sheath poorly developed. Perigonial leaves shorter than stem leaves, consisting mostly of the sheaths, abruptly apiculate. Calyptra without longitudinal split, not persistent on the seta. Capsule ± non–plicate (only immature ones known); exothecial cells with very sinuose walls; stomata on the neck; endostome cilia with numerous blunt appendiculations on the inner surfaces.

Only one sporulating specimen is known (from Alaska), but the capsules are immature; T. sibirica has deciduous leaves which can aid in vegetative dispersal. In wet or moist, predominantly calcareous sites, often near small streams, on seepage slopes, near snowmelt areas, or in depressions in open tundra; frequently intermixed with other mosses, including other species of Timmia; 0--1500 m; Greenland; Alta., B.C., Nfld., N.W.T., Que., Yukon; Alaska; Europe (Norway, Russia).

Timmia sibirica is a very distinct species: the limb lamina and limb costa cells that are mamillose and pleuripapillose on both surfaces are unique within the genus. It is an arctic species, and is rare south of 65° N. T. sibirica also has somewhat larger leaf cells than T. norvegica (and the largest cells of any Timmia taxon).

Version 1: March, 1999