Plants ca. 5 mm high, dull green, in small dense tufts. Stems erect, not or scarcely branched, ca. 0.10–0.15 mm in diameter, pale green, cells in cross section thin-walled throughout, medulla cells 20–25 µm in diameter, epidermal cell layer smaller, ca. 10–15 µm in diameter. Leaves densely placed around stems, erect-patent, long-triangular, 0.7–1.1 mm × 0.15–0.17 mm, gradually tapering from the base into a sharp angled apex; leaf margins smooth, plane; costa percurrent to shortly excurrent, 50–60 µm wide at the base, in cross section flattened, only 20–25 µm thick, stereids absent; leaf cells rather uniform, mid-leaf cells rectangular to hexagonal, ca. 40–60 µm × 7–9 µm, marginal cells not differentiated from the mid-leaf cells, basal cells shorter and wider, 15–35 µm × 12–15 µm. Rhizoids up to 50 µm wide, smooth, pale grayish brown; rhizoidal tubers present at the end of the main rhizoid branches, consisting of a uniseriate row of inflated cells, 0.5–2.0 mm × 100–200 µm; tuber cells isodiametric, subspherical. Sporophytes unknown. (Description of the species is mainly adopted from Arts & Magill 1994.)

Type. China: Yunnan, Xishuangbanna, Meng-la Co., at km marker 637 along Menglun-Mengbang road, north facing wooded slope, R. Magill, P. Redfearn & M. Crosby 7704, 22 December 1986 (holotype MO; isotypes KUN, PC).

Chinese specimen examined: see the type information cited above.

Habitat: on calcareous rocky soils; Distribution: endemic to China.

According to Arts and Magill (1994), Archidium yunnanense is differentiated from most species excepting A. elatum Dix. & Sainsb., A. indicum Hampe & C. Müll., and A. subulatum C. Müll. in the genus based on its leaf shape (widest at base) and strong costa (filling ¼ – 1/3 the leaf width). However, Archidium yunnanense can be separated easily from these related species by its narrow mid-leaf cells, less than 10 µm wide.

Illustrations: Arts & Magill 1994 (Figs. 1, 2).