Hypnum neckeroides Hooker, Musci Exot. Pl. 58. 1818.
Thamnium neckeroides (Hooker) Jaeger. Ber. St. Gall. Naturw. Ges. 1875–1876: 311. 1877.
Porotrichum neckeroides (Hooker) . Williams. Bull. New York Bot. Gard. 2:137. 1901.
Hypnum leibergii Britton. Bull. Torrey Bot Club 16: 111. 1889.
Thamnobryum leibergii (Britton) Nieuwland. Amer Midl. Nat. 5: 51.1917.
Plants in loose mats or cushions, glossy olive- green to yellow-green, dendroid from prostrate stolons but with the branches often so long as to obscure the dendroid habit.. Erect axes to 15 cm long, radially branched with the branches to 5 cm long, often flagelliform at apices. Stipe leaves ovate, appressed, much smaller than branch leaves. Branch leaves strongly concave, to 1.5 mm, 2–3:1, not complanate or plicate, ovate to almost orbicular, abruptly contracted to the obtuse apex, loosely imbricate when dry and erect-spreading when moist. Margins plane, not decurrent, coarsely dentate with large teeth from near the middle to the apex, sometimes serrulate throughout. Median cells smooth, 6–9 µm wide, 2–5:1, thick-walled with lumen/wall ratio to 1:1, but not pitted. Alar cells not differentiated or merely somewhat shorter than adjacent laminal cells. Basal juxtacostal cells narrow, thick-walled (lumen/wall ratio about 1:1), usually strongly pitted. Some of the marginal cells abruptly more narrow and longer than adjacent laminal cells and forming an incomplete limbidium. Apical cells rhomboidal, 1–2:1. Costa tapering from a broad base to a narrow apex, extending to within about ten cells of apex, distally with several abaxial spines and often somewhat flexuose. Pseudoparaphyllia lanceolate, serrate, erect. Axillary hairs 2–3 celled, to 50 µm with one basal brown cell, only slightly offset from leaf insertion. Rhizoids red-brown, 6–8 µm, smooth, inserted in scattered fascicles on ventral surface of prostrate stems.
Dioicous with male plants similar to females. Perigonia and perichaetia on main erect axis. Perichaetial bracts to 3 mm long, convolute around the seta, ecostate, ovate with reflexed, abruptly acuminate apices, cells long rectangular, to 15:1. Seta to 15 mm long, exserted, erect or somewhat bent apically. Capsule brown, erect to inclined, not distorted or sulcate when dry. Urn to 2.0 mm, about 2:1. Exothecial cells l.5–2:1, to 35 µm wide, thick-walled with a lumen/wall ratio of 4–6:1, stomata restricted to neck. Operculum rostrate, about 1/2 as long as urn. Exostome horizontally striate below, becoming low papillose above, brown. Endostome segments papillose, open on keel, narrow, about as long as exostome with basal membrane about 1/3 of total length. Cilia 2–3, well-developed. Spores smooth, 10–12 µm.