Plants slender, whitish green, in cushions or tufts. Stems erect, sparsely to multi-branched, rhizoids abundant at base of stems or at leaf axils and leaf apex; central strand absent. Leaves in three regular ranks or the ranking obscure, hardly twisted when dry; gradually narrowed from an erect, somewhat sheathing, hyaline base to a lanceolate subula; at shoulders of hyaline leaf base sometimes forming cilia or papillose projections; costa thick and broad, convex on dorsal side, concave on ventral side, papillose on both sides, with 4–8 layers of leucocysts enclosing 3 layers of small chlorocysts (dorsal, central, and ventral), chlorocysts in cross section 3–4 sided or drawn out into spiniform or coronate projections; basal hyaline laminal cells in multi-rows, only 1–2 rows toward upper parts, with a few rows of thickened and papillose differentiated cells at middle of leaf; median laminal leucocyst cells mostly quadrilateral or hexagonal, shortening to more or less isodiametric in leaf base and subula. Dioicous. Setae slender, red, smooth, straight to curved, spirally twisted when dry; capsules erect, terminal or pseudolateral, ellipsoid-cylindrical, reddish brown, smooth; opercula conic-rostrate with a long, oblique beak; annuli present; stomata abundant; peristome teeth 16 in 8 pairs, narrowly lanceolate, perforate or thickened with papillose, properistome present. Calyptrae cucullate. Spores finely papillose.

The genus Exostratum was established by Ellis (1985) based on Exodictyon Card. It includes four species in the world. Two species were reported from China. Exostratum sullivantii (Dozy & Molk.) L. T. Ellis reported from Taiwan (M.-J. Lai & J.-R. Wang-Yang 1976; C.-M. Kuo & T-Y. Chiang 1987) could not be confirmed at this time. According to Ellis (1985), plants of E. sullivantii are similar to those of E. blumii (Nees ex Hampe) L. T. Ellis. The genus was placed in the subfamily Leucophanoideae of the Calymperaceae by Ellis (1985). However, it is placed here under the Leucobryaceae in the traditionally broad sense dealing with leucobryoid mosses.

Lower Taxon

• Exostratum blumii (Nees ex Hampe) L. T. Ellis