Musci Cleist. p. 19, 1869. -- Acaulon muticum var. rufescens (Jaeg.) Crum, Bryologist 72: 240. 1969.
Plants glossy, yellow-green to golden, mostly scattered or somewhat gregarious and bulbiform. Leafy stems to 2 mm high with the stem essentially obsolete. Leaves concave, not or inconspicuously keeled, erect and enclosing the capsules, not contorted when dry. Leaves ovate to very short-elliptic, 1.5–2: 1, scale-like below but grading imperceptibly to the perichaetial leaves which may exceed 1.5 mm in length. Lamina unistratose throughout. Median and distal cells only gradually differentiated from the basal cells. Median laminal cells elliptic to hexagonal, to 12 µm broad, 1.5–2: 1, with elliptic lumens and without defined corner thickenings. Basal cells rectangular with straight lateral walls, to 14 µm broad, 3–4: 1, smooth, moderately thick-walled, unistratose throughout. Costa narrow from its base to its percurrent to short excurrent, reflexed apex. Costa cross-section at mid-leaf about twice at wide as thick, only slightly thicker than the adjacent lamina, with only a slight hint of an abaxial stereid band, otherwise essentially homogeneous. Margin crenulate-serrulate or occasionally nearly entire, often with irregular dentations near apex, recurved to well above the middle. Axillary hairs pale brown and rather thick-walled, to 120 µm long, 3-celled, with no basal brown cells. Rhizoids hyaline, smooth to minutely verruculose, sparingly branched, to 20 µm wide at insertion.
Dioicous with the male plants so small as rarely to be seen. Perichaetial leaves larger than, but only gradually demarcated from the vegetative leaves. Seta mostly less than 0.5 mm long, straight. Capsule globose, to 0.5 mm in diameter, apiculate at apex but without a differentiated operculum. Calyptra mitrate, covering only the apiculus. Peristome absent. Exothecial cells mostly short rectangular, 2–3: 1, with curved lateral walls, 15–20 µm wide. Stomates phaneroporous, restricted to base of urn. Spores 30–50 µm, spherical to very short elliptic, nearly smooth.