Man. Moss. W. Pennsylvania 287. 1913. -- Hypnum luridum Hedw., Spec. Musc. 291. 1801.
Hypnum pseudoarcticum Kindb. in Mac., Bull. Torr. Bot. Club 17: 280. 1890.
Hypnum columbico-palustre C. Müll. & Kindb. in Mac., Cat. Canad. Pl. 6: 241. 1892.
Plants to 40 mm long, in lustreless, olive-green to green mats. Stems weak, green when young, turning brown with age. Plants prostrate to weakly decumbent, irregularly branched with little differentiation of stems and branches. Leaves usually rather remotely inserted on stem, ovate to ovate lanceolate with bluntly rounded apices, to 1 mm long, about 2: 1, not at all falcate, weakly ascending, little changed wet or dry. Margins plane and entire throughout, usually slightly reflexed into the small decurrency. Median laminal cells of well-developed leaves linear-vermiculose, 4–6: 1, to 6 µm broad, smooth, rather thick-walled with lumen: wall ratio 3–5: 1. Cells of alar region sparingly prorate, quadrate to short rectangular in a small group of not at all inflated cells to 10 µm wide, 1–2.5: 1, mostly to 4 cells broad and extending about three cells up the margin and several cells into the slight decurrency. Marginal cells, especially in proximal half of leaf, broader and more pellucid than adjacent interior cells. Cells in a small group at leaf apex quadrate to short rectangular 1–3: 1, thick-walled with lumen: wall ratio mostly less than 2: 1. Costa slender, about 6 cells wide at the base, mostly forked from near the base, with both forks ending below the leaf middle. Pseudoparaphyllia filamentose, to 3 cells long, closely clustered to 30 µm long. Axillary hairs about 6-celled, of uniform diameter throughout, thin-walled, to 100 µm long, with two of the basal cells short but concolorous, not offset from leaf insertion. Stem cross-section with a strong hyaloderm, a stereome 3–4 cells thick with those thick-walled cells abruptly demarcated from an inner medulla of thin-walled and larger hyaline cells with central strand usually present occasionally indistinct. Rhizoids scarse even on the older prostrate axes, fasciculate at abaxial leaf insertions, to 18 µm at insertion, pale-brown to brown, sparingly branched, smooth.
Autoicous. Perichaetia placed on older prostrate stems with the outer bracts short and squarrose but with the inner bracts to 2 mm long, erect and loosely sheathing the base of the seta. Inner perichaetial bracts doubly costate to above the middle, plicate, elliptic-lanceolate with parallel margins in proximal 1/2, mostly rather brownish-green, gradually contracted at about the middle to the bluntly acute apex with no alar differentiation and with margins nearly entire and smooth throughout. Seta to 10 mm long, smooth, erect, red-brown. Urn red-brown, to 1.2 mm long, about 1.5: 1, oblong, curved and inclined, not sulcate but lightly strangulate. Exothecial cells moderately thick-walled, not collenchymatous, not in regular rows, to 30 µm broad, mostly elongate hexagonal, 2–4: 1, not strongly differentiated near the mouth. Stomata few, short and vertical pored, on lower portion of urn. Operculum conic, to 1/5 as long as urn. Annulus absent. Calyptra cucullate and smooth, sheathing only distal half of capsule. Exostome teeth inserted and somewhat united near mouth of urn, to 500 µm long, lanceolate but with a prominent shoulder at about middle, transversely striate, inflexed when dry, pale yellow to yellow, with a well defined zig-zag median commisural line. Endostome segments papillose, nearly as long as exostome, consisting of keeled, narrow segments with 2–3 cilia, arising from a basal membrane about 1/2 as long as exostome. Spores to 18 µm, finely papillose.