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DescriptionThallus: usually not evident as a distinct structure, but basidiomata originating from tangled hyphae which in some species form a dark greenish, crust-like growth that may be visible even when basidiomata are absent; Basidiomata: "crustose" (forming a matt-like cover of appressed or erect branched fibrils) to "foliose", often forming semicircular brackets, which are sessile, standing out from the substrate, or resupinate (partly attached, partly lifted up and bent backwards), single or united in rosettes, soft or paper-like, small or to 20 (-25) cm diam.; upper surface: blue- to gray-green, gray to dark or olive green, or whitish or yellowish, often with concentric markings, sometimes unevenly thickened or sulcate (grooved) or zoned, smooth to frequently roughened, or becoming villose, hispid or fibrillose (appearing "combed" hairy or shaggy) due to irregularly to radially arranged filaments of the photobiont; photobiont: primary one a cyanobacterium (Scytonema or sometimes Chroococcus or similar genera), secondary photobiont absent, usually in a well-developed layer in the basidiomata, the filaments surrounded by a coating of longitudinally arranged hyphae; lower surface: smooth and even, to warty or granular, without gills or pores; hymenophores (hymenium-bearing parts) soon or eventually produced, scattered to reticulate or forming low concentric bands, dehiscent, white, cream or buff, smooth to ± tomentose, continuous or broken up; basidiomata anatomy: with monomitic hyphal system, consisting only of diploid (dikaryotic) generative hyphae (giving rise to other hyphal types and to the hymenium) which are thin- to thick-walled, hyaline or yellowish, septate, irregularly branched or at times dichotomous, with or without clamp connections , 3-11 (-13) µm diam.; without sterile bodies (cystidia and gleocystidia); hymenium organized into an irregular but definite palisade layer with thick increments of basidia and basidioles (immature basidia); basidia: in bundles, clavate or subcylindrical, not constricted, 15-30 x 5-9 µm, bearing four slender, slightly curved sterigmata (spore-producing structures); basidiospores: simple, hyaline or yellowish brown, ellipsoid to subcylindrical or almost boat-shaped, smooth, I-, 6-10 x 2.8-5 µm, thin-walled, non-amyloid; Secondary metabolites: none detected; Geography: mostly tropical or subtropical, but some species occurring in boreal-temperate areas; Substrate: trees, mossy rocks and soil.; Notes: Although this genus lacks a true thallus (in the sense of a distinctly developed "vegetative plant body"), some authors have referred to the basidioma, which contains the photobiont, as being the thallus.