Hypnum robustum Hook., Musci Exot. 108. 1819. -- Hylocomium robustum (Hook.) Kindb., Enum Bryin. Exot. 1888.
Hylocomium flemingii Aust., Bull. Torr. Bot. Club 5: 24: 1874.
Plants to 15 cm long, in pale-green or yellowish-green wefts or mats, irregularly and sparingly branched. Leaves plicate and lightly undulate, closely overlapping, to 5 mm, about 3: 1, ovate to ovate lanceolate, gradually contracted to the acumen. Margins recurved near base or with that recurvature extending nearly to leaf middle, not at all decurrent, entire to serrulate at base, serrulate to serrate above. Median cells smooth, 4–5 µm wide, linear, about 8–15: 1, thick-walled with lumen: wall ratio 2–3: 1, strongly pitted. Median basal cells mostly concolorous, broader and shorter than adjacent, more distal cells, thick-walled with lumen: wall ratio about 1: 1. Alar cells somewhat differentiated in a rounded rather pellucid pocket, short-rectangular, 2–3: 1, strongly pitted, to 10 µm broad. Costa double and separate from base, symmetrically inserted in center of leaf base, with both forks extending above the middle. Paraphyllia to 600 µm long, cloaking most of stem except for the most immediate cortical cells abaxial to leaf insertion, one cell wide except at extreme base, once to twice branched with branches somewhat paired. Axillary hairs 5–7 celled, to 250 µm with two basal cells brown and short, mostly offset from leaf insertion. Stem cross-section with a small central strand, without a hyaloderm, with a well marked stereome of pachydermous, pigmented cells, and with inner cortical cells larger and mostly rather thin-walled. Rhizoids very rarely produced on old and prostrate stems or on bases of perichaetia, to 25 µm in diameter at insertion, arising from area immediately lateral to alar cells at leaf insertions, red-brown, monopodially branched with branches short, smooth and rather thick-walled.
Dioicous with male plants similar to females. Perichaetia on old, prostrate axes, with bracts squarrose and markedly shorter (to 1.5 mm) than the subtending leaves. Seta to 3 cm long, erect, redddish brown, smooth. Capsule red brown, inclined and arcuate, neither sulcate nor strangulate. Urn to 1.5 mm, oblong and asymmetric, about 2: 1. Exothecial cells isodiametric, quadrate to hexagonal, to 30 µm broad, in regular rows, with corners somewhat thickened. Stomata phaneroporous, restricted to neck. Annulus present, not revoluble. Operculum conic, much shorter than urn. Exostome to 500 µm long, horizontally striate throughout or somewhat papillose near the apex, gray-brown. Endostome segments lightly papillose, perforate along keel, about as long as exostome with basal membrane about 2/3 of total length. Cilia 1–3, lightly papillose, somewhat nodose. Spores green, smooth to very lightly papillose, to 15 µm.