Plants 5--10 mm, dark black-green. Stem leaves ovate-lanceolate, (0.8)1.5--1.8 ´ (0.3)0.6--0.8 mm, margins incurved distally, apex often cucullate, awn 0.0(0.3)--0.5 mm, lamina 2-plicate; distal laminal cells 2-stratose; medial laminal cells patchy 2-stratose; juxtacostal basal laminal cells short-rectangular, 19--33 ´ 9--13 µm, evenly thick-walled; marginal basal laminal cells quadrate to short-rectangular, 13--27 ´ 9--13 µm, evenly thin-walled. Sexual condition dioicous. Seta 0.3--0.5 mm. Capsule immersed, ovoid to campanulate; peristome absent.

Subspecies 2 (1 in the flora).

Mineralized siliceous rock; 160 m; N.W.T.; Asia (Pakistan, Mongolia).

Until 1996, Coscinodon arctolimnius was considered to be endemic to the eastern shore of Great Bear Lake in the Northwest Territories of Canada. W. C. Steere (1974) suggested that the species evolved in this site as a result of radiation induced genetic mutations on a population of C. cribrosus. However, with the discovery of a new subspecies (R. I. Hastings and H. Deguchi 1997), C. arctolimnius subsp. higuchi, from both Pakistan and Mongolia, it appears that the species has a much wider distribution. Although undoubtedly closely related to C. cribrosus, there is now no reason to assume that the species evolved in North America. The Asian subspecies is distinguished in part by a smaller leaf size, longer awns, more commonly cucullate apex, and details of the structure of the plications. Coscinodon arctolimnius is separated from other North American Coscinodon by its green-black color, cucullate leaf apex (incurved if awned), and by the absence of a peristome.