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DescriptionThallus: orbicular to irregular, to 5 cm diam.; lobes: up to 2 mm broad, flat to convex, eciliate; upper surface: whitish gray to dark gray, rarely brownish gray, usually distinctly white-maculate, sometimes weakly white-pruinose; without soredia or isidia; medulla: white; lower cortex: prosoplectenchymatous with the lowermost part gradually forming rounded, isodiametric cells; lower surface: white to dark lead colored; Apothecia: usually abundant, up to 2 mm diam.; disc: often white-pruinose; ascospores: brown, 1-septate, vary from Physcia- to Pachysporaria-type, (19-) 21-25 (-28) x 9-11 (-13) µm; Pycnidia: common, immersed; conidia: subcylindrical, 4-6 x 1 µm; Spot tests: cortex and medulla K+ yellow, C-, KC-, P+ yellow; Secondary metabolites: upper cortex with atranorin; medulla with atranorin, zeorin and rarely other triterpenes.; Substrate and ecology: growing on trees in fairly open situations; World distribution: widely distributed within arctic, boreal and temperate areas of North American and Eurasia, also known from South America; Sonoran distribution: widely distributed from sea level to upper elevation montane sites from Arizona, southern California, Baja California, Baja California Sur, Chihuahua, Sonora and Sinaloa.; Notes: Some material seen from the Sonoran Region is a somewhat different from "typical" P. aipolia, but the difference is not so obvious that a new species is described. The whole pattern of variation in North America has to be studied further.