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DescriptionThallus: minutely foliose, lobes coalescing to form a crust-like cushion, 1-2 cm wide; lobes: irregular, flattened, elongate, imbricate, often dissected (torn?); main branches 1-2 mm wide; dissected branches: 0.1-0.2(-2) mm wide, (45-)80-100 µm thick; apices: developing long cylindrical to coralloid isidial-like appendages; surface: medium gray to brownish gray or black, smooth to granular, not isidiate but with clusters of coralloid outgrowths; internal anatomy: with upper and lower cortices consisting of irregularly isodiametrical cells 12-16 µm in diam., internally paraplectenchymatous and with loosely interwoven chains of Nostoc; Apothecia: common, laminal, sessile, 0.1-0.8 mm wide; disc: light to dark brown to black, concave to plane; margin: thalline, concolorous with the thallus or paler, entire; exciple: euparaplectenchymatous but difficult to discern, 10-20 µm thick centrally; hymenium: hyaline below and thinly yellow to brown above, 105-180 µm tall; paraphyses: unbranched, c. 1 µm wide, slightly inflated apically; subhymenium: pale gray, 25-35 µm thick; asci: cylindrico-clavate, 8-spored; ascospores: hyaline, submuriform to muriform, 3-7-septate transversely, 0-2-septate longitudinally, ellipsoid to subfusiform, 17-37 x 9-14 µm; Pycnidia: not observed; Spot tests: all negative; Secondary metabolites: none detected.; Substrate and ecology: among mosses on calcareous soil; World distribution: Europe, Asia and North America, especially in arctic areas; Sonoran distribution: Arizona, southern California and Baja California (Guadalupe Island).; Notes: Previously Leptogium tenuissimum was confused with L. subtile, a much smaller species with smaller, globose apothecia and terete thallus lobes.