Plants isolated or clumped in loose patches, 5--10(-- 15) mm. Stems of main sterile shoots and fertile male plants 0.2--0.3 mm wide; fertile shoots of female plants to 0.5 mm wide; rhizoids numerous, long, weakly clumped below the erect apex, clear to brown or faintly purple. Lateral leaf insertion dorsally transverse with short decurrence along the dorsal midline of axis, lateral insertion long-oblique, ventral insertion transverse; margins frequently decolorous in narrow zone. Sexual condition dioicous, male and female plants forming separate, often widely spaced patches, rarely occurring together. Androecia typically comprising several pairs of tightly imbricate androecial bracts, the inflorescence becoming intercalary with apical growth, often forming fertile branches with successive male inflorescences interrupted by a few vegetative leaves. Gynoecia 2-valvate, labial bracts with weak dorsiventral compression at maturity, interior bracts bearing slime papillae along labial contact surfaces; perianth bearing a few scattered marginal slime papillae. Sporophytes (5--)8--10(--15) mm at maturity. Capsule with 12--18 distinct longitudinal ridges.

Capsules mature in summer (July--Aug.). Mature sporophytes are known only from a single collection. Generally restricted to perennially moist basic substrates, peaty soil over basalt, slopes influenced by meltwater from basalt cliffs, calcareous fens, basic mineral soil and calcareous silt deposits, margins of shallow lakes; 0--600 m; Greenland (R. M. Schuster 1969, 1988); Nun. (Ellesmere Island); n Alaska (Brooks Range), Mich., Minn.; Europe (Norway, Sweden); Asia (Russia in Siberia).

This rare circumarctic species is disjunct in a narrow tundra zone along shorelines of a few islands in northern Lake Superior--Susie I., Minnesota, and Apostle I., Michigan (R. M. Schuster 1969), and should be sought in similar refugial habitats elsewhere in the boreal latitudes. Known outside the flora area from Chukoskiy Peninsula in Siberia (R. N. Schljakov 1975), Spitzbergen, and Swedish Lapland (J. A. Paton 1999). Cryptocolea imbricata is most likely to be confused with Arnellia fennica. Though similar in habit, size, leaf-shape, and color, Arnellia is never shiny, has distinctly opposite lateral leaves, and ventral leaves that are easily revealed. A few misidentified collections are Nardia geoscyphus, which, though also laterally compressed, is distinctly smaller in all respects, and has prominent trigones and well developed ventral leaves.