Bryol. Univ. 2: 81. 1827 ,.
Plants in soft, dense tufts, cushions or sods, yellow to dark green distally, pale-brown proximally, or sordid brown throughout, occasionally with a red tinge in young leaves. Stems 0.5–7 (–10) cm, often sparingly branched by subfloral innovations, smooth or occasionally scabrous-papillose; transverse-section round, rounded-pentagonal, or triangular, hyalodermis usually absent, sclerodermis present, central strand absent [present]; moderately radiculose proximally, occasionally to the apex; axillary hairs of around 8 cells, the basal 1–2 cells brownish or occasionally all cells hyaline. Leaves appressed to appressed-incurved and weakly contorted when dry, weakly to strongly spreading-recurved when moist; linear to oblong-lanceolate, adaxial surface sharply keeled or canaliculate, 0.5–2.8 (–3) mm; base scarcely differentiated to ovate or rectangular, sometimes semi-sheathing proximal to a reflexed distal lamina; distal margins narrowly recurved along 1 or both sides, or margins of expanded base broadly recurved, all margins papillose-denticulate to smooth or scalloped-denticulate, occasionally 2-stratose distally in small to extensive patches, apex sharply to bluntly acute, sometimes apiculate with a clear cell, acuminate-mucronate, or rarely obtuse to rounded; costa ending 2–6 cells before the apex or percurrent to excurrent as a stout mucro, costal adaxial cells epapillose, in 4–5 rows, abaxial cells elongate, 3: 1, papillose or smooth, transverse-section semicircular or concave adaxially, adaxial costal epidermis absent, adaxial stereid band present but absent in depauperate plants, guide cells 2–4 (–6) in one layer, hydroid strand absent, abaxial stereid band well developed, abaxial epidermis absent or weakly differentiated; basal-cells differentiated, typically firm to occasionally lax medially, rectangular, 2–4: 1, even or often strongly irregularly porose, with narrower rectangular cells extending into a slender, often fragile decurrency; distal laminal cells typically heterogeneous, larger along the costa grading to distinctly smaller on the margins, irregularly subquadrate to shortrectangular to porose throughout, 8–12 µm wide, 1–2 (–3):1, cell-walls thin to typically incrassate, flat to slightly convex in section, papillae simple and sharp to 2-fid, 1–3 or more per lumen, centered to scattered, occasionally absent, lumina clear to pellucid. Specialized asexual reproduction none. Sexual condition dioicous; perigonia gemmate, terminal; perichaetia terminal, leaves little different from the cauline, somewhat longer, slightly sheathing. Seta yellow when young, redbrown with age, 0.5–1 cm, twisted clockwise. Capsule yellow to redbrown, variably globose, ovoid, obovoid, or subcylindric, ca. 0.5–1 mm; operculum inconstantly systylius, obliquely conic-rostrate, 0.2–1 mm; peristome absent. Calyptra cucullate, smooth. Spores ca. 17 µm, brownish, finely papillose. KOH laminal color reaction yellow.
Species 15 (1 in the flora).
Aside from the gymnostomous capsule, the most important character distinguishing Hymenostylium from other genera in the flora area is the absence of a stem central strand. Gymnostomum, with a similar habitat, has a central strand. Species in the peristomate genus Hennediella, for example, also may have systylius capsules but have central strands in addition to a laminal marginal border, while the costal adaxial stereid band is absent. The genus Trichostomum has a stem central strand, as is characteristic of Barbula and Didymodon. Leptodontium is similarly without a stem central strand, has a strong sclerodermis and hyalodermis, and reduced costa in cross section, but is distinct by the well-developed peristome, toothed leaf margin and nonhygric habitat. Eucladium, also without a central strand, lacks a sclerodermis, and its costa has both an ab- and adaxial epidermis and two stereid bands. Hymenostylium is frequently if perhaps not accurately described and illustrated as having no hyalodermis. However, a large hyalodermis was often present in specimens from the flora area, this usually collapsed and perhaps overlooked. According to R. H. Zander (1993), Hymenostylium is phylogenetically related to Leptodontium in the Leptodontieae, whereas Gymnostomum, a genus into which Hymenostylium is often placed, belongs with the Barbuleae.