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DescriptionThallus: squamulose, approximately circular to oblong in outline, small or medium-sized, up to 2 cm in diam., or composed of overlapping, fan-shaped lobes, 0.5-2 cm wide; lobes: +flattened, separate or imbricate; margin: rounded, entire to slightly indented, flat; upper surface: gray-green when dry (becoming pale to dark brown in the herbarium), bright green when wet, smooth, +shiny, etomentose and epruinose, without isidia or soredia; medulla: white, with +loosely interwoven hyphae; photobiont: primary one Coccomyxa and secondary Nostoc in cephalodia; lower surface: white to pale brown, with conspicuous, fan-shaped, pale to dark brown veins, tomentose, erhizinate, cephalodiate; cephalodia scattered, associated with the veins, superficial, dark brown or gray, rhizinate; Apothecia: frequent, 1-6 per squamule, +round to oval, sessile, marginal, up to 4 mm in diam.; disc: flat, brown-black, smooth; ascospores: colorless, fusiform, 3-septate, 30-47 x 7-8 µm; Pycnidia: not seen; Spot tests: all negative; Secondary metabolites: tenuiorin, methyl gyrophorate, gyrophoric acid, phlebic acids A & B, and terpenoids.; Substrate and ecology: among mosses over soil in moist habitats at relatively high elevations; World distribution: temperate and boreal regions of North America, Europe and Asia; Sonoran distribution: occasional at higher elevations in Arizona.; Notes: Its small, green thalli with fan-shaped non-rhizinate venation and its flat round apothecia, that are practically always present, make P. venosa the easiest species in the genus to identify.