Limnobium norvegicum Schimper in P. Bruch and W. P. Schimper, Bryol. Europ. 6: 70, plate 576. 1853; Calliergon viridulum (Lindberg) Kindberg; L. viridulum (Lindberg) Kindberg; Ochyraea norvegica (Schimper) Ignatov & Ignatova

Plants soft, pale yellow-green, light brown with age. Stems to 1.5(-3) cm, somewhat denuded basally, irregularly branched; hyalodermis absent, epidermal cells small, walls thick, similar to subadjacent cortical cells, central strand present. Leaves erect-spreading to imbricate, straight, sometimes secund, crisping to shrinking laterally and concomitantly involute and twisting when dry, straight and loosely imbricate to spreading when moist, usually ovate, less often broadly ovate, plane to shallowly but clearly concave, (0.4-)0.5-0.8(-0.9) × (0.2-)0.4-0.5(-0.6) mm; margins usually plane, rarely narrowly recurved basally, rarely to shoulder, entire; apex acute, occasionally obtuse, sometimes slightly squarrose; costa short, double; alar cells few, quadrate to short-rectangular, region undifferentiated or small, irregular; basal laminal cells variable; medial cells short-rhombic, bacilliform, fusiform, or linear-flexuose, (16-) 20-30(-48) × 5-6 µm; apical cells shorter; marginal cells rarely longer than 55 µm. Sexual condition autoicous; ; perichaetial inner leaves lanceolate, . Seta yellow to reddish, 0.6-1 cm. Capsule with endostome cilia 2 or 3.

Irrigated acidic rock in montane streams; moderate to high elevations (600-1700 m); Greenland; B.C.; Alaska; n Europe.

Hygrohypnum norvegicum is a poorly understood species; eight North American specimens are too few from which to develop a sound concept of its distinctiveness and variability. The lone specimen from British Columbia looks like a very small H. molle. There are some very small specimens of H. cochleariifolium with which the British Columbia specimen could be confused. Hygrohypnum norvegicum is distinguished by its loosely imbricate to spreading, tiny, ovate leaves with acute apices. The plants grow in loosely to tightly woven patches.