Hypnum luridum Hedwig, Sp. Musc. Frond., 291. 1801; Calliergon pseudomontanum (Kindberg) Kindberg; Hygrohypnum palustre (Hedwig) Loeske; H. palustre var. julaceum (Schleicher ex Bruch & Schimper) Loeske; H. palustre var. subsphaericarpon (Schleicher ex Bridel) Loeske; H. pseudomontanum (Kindberg) Grout; Hypnum columbicopalustre Müller Hal. & Kindberg; H. palustre Hudson ex Bridel; H. pseudomontanum Kindberg; H. subeugyrium var. occidentale Cardot & Thériot; Limnobium pseudomontanum (Kindberg) Kindberg; Scleropodium krausei (Müller Hal.) Macoun & Kindberg

Plants soft, yellow-green, sometimes with rusty mottling, or less often yellow-brown, bright green, or blackish green, regularly grading from one color to another within same plant. Stems to 6 cm, usually leafy throughout, irregularly branched; hyalodermis absent, epidermal cells small, walls thick, similar to subadjacent cortical cells, central strand well developed. Leaves usually imbricate, often julaceous, straight or falcate, rarely almost circinate, little different when dry or moist, often oblong-lanceolate, lanceolate, ovate, or occasionally broadly ovate, not deeply concave, (0.5-)1-1.5(-2.5) × (0.3-)0.4-0.8(-1.1) mm; margins regularly involute, not recurved at apex, entire; apex acute or slightly short-apiculate; costa single and short to percurrent, double and short, or ecostate, often within same plant; alar cells many, quadrate, short-rectangular, or irregular, , region usually clearly differentiated; basal laminal cells wider, shorter than medial cells; medial cells short-rhombic to linear-flexuose, (28-)30-55(-95) × (4-)5-7(-12) µm; apical cells shorter; marginal cells rarely longer than 55 µm. Sexual condition autoicous; perichaetial inner leaves lanceolate, . Seta yellow-brown, 0.8-2.4 cm. Capsule with endostome cilia 1-3.

Irrigated calcareous rock, stone, wood, in montane streams; moderate to high elevations (400-3200 m); Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., Nunavut, Ont., Que., Sask., Yukon; Alaska, Ariz., Calif., Colo., Idaho, Mich., Minn., N.H., N.J., N.Mex., N.Y., Ohio, Utah, Vt., Wash., W.Va., Wyo.; Europe; Asia.

Hygrohypnum luridum is a highly variable species in which many infraspecific taxa have been recognized; there are, however, no reliable distinguishing features among them. In North America, three forms are regularly seen, to which no taxonomic recognition is given here. Two have falcate leaves and differ essentially in size, one large and one small. A third form has leaves that are essentially straight and vary in stance from imbricate to julaceous. Complicating this is the fact that the small, falcate-leaved forms intergrade with those having straight leaves and may occur as alternating regions on the same stem. In all cases, however, the costa varies from double and short to single and either short or long and is coupled with a well-defined group of quadrate, short-rectangular or irregularly shaped alar cells. In large, falcate-leaved forms in the West, the costa is often strong and single to almost percurrent. Hygrohypnum luridum is the only calcicole in the genus; it occurs on calcareous rock or on other substrates irrigated with calcareous water.