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DescriptionThallus: foliose, up to 10 cm in diam., usually ± orbicular; lobes: linear and discrete to somewhat irregularly flabellate and confluent or weakly imbricate, 1-2 (-3) mm broad, ± flat and prostrate; upper surface: gray to gray-brown or brown, often with a ± complete pruina or at times the pruina best developed on the lobe ends; without soredia or isidia, but occasionally developing irregular sparse to rather abundant lobules centrally; upper cortex: paraplectenchymatous; medulla: white; lower cortex: irregularly prosoplectenchymatous; lower surface: pale tan to pale brown along the peripheral lobes (sometimes up to 4-6 mm from tip), darkening to dark brown or black inwardly, dull to weakly shiny; rhizines: black, mostly squarrosely branched; Apothecia: frequent and nearly always present, up to 3 mm in diam., the margin entire or more often developing lobules, pruinose (including discs, margins, and lobules); ascospores: brown, 1-septate, 26-36 (-38) x 15-19 µm; Spot tests: all negative; Secondary metabolites: none detected.; Substrate and ecology: on bark (esp. deciduous trees or shrubs), wood, or occasionally rock; World distribution: western North America, southern Europe and northern Africa, Asia; Sonoran distribution: southern California and Isla Cedros (Baja California).; Notes: There are two other Physconia species occurring in the Sonoran region which also lack soredia and isidia, and which might be confused with P. americana. Physconia muscigena can be distinguished by the concave, upturned peripheral lobes, the frequent development of similarly erect secondary lobules (at times numerous and turf-forming), and the usual substrate (moss or soil, usually over rock). Physconia californica is a similar, primarily corticolous species, which differs by having a more uniformly pale, white to tan, lower surface and a regularly lobulate upper surface. Physconia americana was long referred to as P. distorta or one of its synonyms, but that species is absent from North America.