6. Hygrohypnum duriusculum (De Notaris) D. W. Jamieson, Taxon. 29: 152. 1980.
Limnobium duriusculum De Notaris in F. Ardissone et al., Erb. Crittog. Ital., ser. 2: no. 204. 1869; Calliergon circulifolium (Müller Hal. & Kindberg) Kindberg; C. dilatatum (Wilson) Kindberg; C. pseudoarcticum (Kindberg) Kindberg; Hygrohypnella duriuscula (De Notaris) Ignatov & Ignatova; Hygrohypnum dilatatum (Wilson) Loeske; H. eugyrium var. dilatatum (Wilson) Grout; Hypnum circulifolium Müller Hal. & Kindberg; H. dilatatum Wilson; H. pseudoarcticum Kindberg; L. pseudoarcticum (Kindberg) Kindberg; Ochyraea duriuscula (De Notaris) Ignatov & Ignatova
Plants coarse, stiff, bright yellow-green, dull olive green, dirty yellow, or yellow-brown, with or without rusty mottling. Stems to 7 cm, leafy throughout or commonly denuded basally, irregularly branched; hyalodermis absent, epidermal cells small, walls thick, similar to subadjacent cortical cells, central strand strong. Leaves often secund, straight, shrinking, spreading, or contorted and twisted to the left when dry, appressed-imbricate and sometimes secund or spreading and somewhat contorted when moist, very broadly ovate, oval, orbicular, or less often ovate, plane or shallowly concave, (0.8-)1.1-1.7(-2.2) × (0.6-)0.8-1.3(-1.7) mm; margins plane or narrowly recurved basally, sometimes involute when dry, usually entire, rarely denticulate; apex acute, obtuse, or short-apiculate; costa double, short, often to mid leaf, sometimes single and 2-fid to beyond mid leaf; alar cells quadrate, short-rectangular, or irregular, , region clearly differentiated, irregular; basal laminal cells wider, longer, ; medial cells short- to long-fusiform or linear-flexuose, (26-)30-70(-90) × (3-)5-7(-9) µm; apical cells shorter; marginal cells rarely longer than 55 µm. Sexual condition autoicous; . Seta yellowish red, red, or maroon, 0.9-2.4 cm. Capsule with endostome cilia 1-3.
Irrigated, emergent acidic rock in montane streams; moderate to high elevations (200-3600 m); Greenland; Alta., B.C., Nfld. and Labr., N.S., Ont., Que.; Alaska, Ariz., Calif., Colo., Conn., Idaho, Maine, Mass., Mont., Nev., N.H., N.Mex., N.Y., N.C., Tenn., Utah, Vt., Va., Wash., Wyo.; Europe; Asia.
Hygrohypnum duriusculum is at the center of a confusing group of species including H. alpinum, H. bestii, and H. molle. These species may be distinguished as follows: H. alpinum is a western species with leaves typically orbicular and alar cells forming a rectangular group of more or less rectangular or irregular, enlarged, thin-walled cells; H. bestii stands alone in having exceedingly long marginal laminal cells, generally exceeding 60 µm; H. duriusculum exhibits very broadly ovate to oblong-elliptic, oval to orbicular leaves, and alar cells forming a well-defined broad group of quadrate, short-rectangular, or irregular, often excavated cells with thickened, regularly discolored walls; H. molle occurs only in the western part of the flora area and has leaves mostly ovate to broadly ovate with an acute but blunt apex, alar cells essentially undifferentiated from surrounding cells, leaves little changed from moist to dry conditions and more or less straight and loosely imbricate. It should be noted that while H. duriusculum occurs across the continent, the plants from western North America are somewhat smaller than those from the east.