Taxon 29: 152. 1980. -- Limnobium duriusculum DeNot., Erbar. Critt. Ital. ser. II. no. 204. 1869.
Hygrohypnum dilatatum (Wils.) Loeske, Moosflor. Harz. 320. 1903. -- Hypnum dilatatum Wils. in Schimp., Syn. 2: 776. 1876.
Plants to 100 mm long, in lustreless, olive-green to sordid-green wefts usually with a russet wash in exposed areas. Stems rigid, green when young, turning almost black with age. Plants initially prostrate but becoming almost erect or pendent in water because of crowding, sparingly branched with little differentiation of stems and branches. Leaves broadly ovate to oblong elliptic with bluntly rounded apices, to 2 mm long, 1.2–1.4: 1,somewhat to not at all falcate, diverging from stem at more than a 45 degree angle when wet or dry. Margins plane and entire but usually with basal recurvature into the 3–5 cell long decurrency. Median laminal cells of well-developed leaves linear-vermiculose, 8–12: 1, to 6 µm broad, smooth, thick-walled with lumen: wall ratio about 1: 1. Cells of alar region short rectangular in a well-demarcated pellucid group to 30 µm wide, mostly about 2: 1 but with the immediate marginal row about half as wide as the adjacent interior cells. Marginal cells, especially in proximal half of leaf, somewhat longer and more pellucid than adjacent interior cells. Cells in a large group at leaf apex short rectangular 2–4: 1, thick-walled with lumen: wall ratio mostly less than 1: 1. Costa slender, of fewer than ten cells wide at base, forked from near the base, with both forks ending near the leaf middle or below. Pseudoparaphyllia broadly ovate and rounded at apex, closely folded over bud, to 400 µm long. Axillary hairs 5-celled, of uniform diameter throughout, thin-walled, to 250 µm long, with two of the basal cells short but concolorous, not offset from leaf insertion. Stem cross-section with a stereome 2–4 cells thick with those thick-walled cells abruptly demarcated from an inner medulla of thin-walled and larger hyaline cells with central strand usually obvious. Rhizoids scarse even on the older prostrate axes, fasciculate at abaxial leaf insertions, to 12 µm at insertion, red-brown, sparingly branched, smooth.
Autoicous. Perichaetia placed on older prostrate stems with the outer bracts short and squarrose but with the inner bracts to 3 mm long, erect and closely sheathing the base of the seta. Inner perichaetial bracts weakly costate to about leaf middle, plicate, narrowly elliptic lanceolate, mostly rather dark brownish green, gradually contracted from base to the acute apex with no alar differentiation and with margins nearly entire. Seta to 15 mm long, smooth, erect, pale- to red-brown. Urn pale-brown, to 1.5 mm long, about 2: 1, oblong-cylindrical, curved and inclined, not sulcate but lightly strangulate. Exothecial cells rather thin-walled, not collenchymatous, not in regular rows, to 20 µm broad, mostly rounded hexagonal, not strongly differentiated near the mouth. Stomata few, short and vertical pored, on lower portion of urn. Operculum conic to inconspicuously apiculate, to 1/5 as long as urn. Annulus present, not revoluble. Calyptra cucullate and smooth, sheathing only distal half of capsule. Exostome teeth inserted and somewhat united near mouth of urn, to 300 µm long, lanceolate but with a prominent shoulder at about middle, transversely striate, inflexed when dry, yellow to orange with a well defined zig-zag median commisural line. Endostome segments papillose, nearly as long as exostome, consisting of keeled, narrow segments with 2 cilia, arising from a basal membrane about 1/2 as long as exostome. Spores to 18 µm, finely papillose.