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DescriptionThallus: foliose, orbicular to irregular, up to 3 cm diam.; lobes: up to 1 mm wide, sometimes scale-like; margin: mostly crenulate when soralia not present, eciliate; upper surface: whitish gray to dark gray or cream-colored (with darker margins), shiny and epruinose or rarely weakly pruinose, sorediate; soredia: in marginal or terminal soralia that develop from the lower surface of the lobe tips, eroding the tip and sometimes also the lower surface; upper cortex: paraplectenchymatous with rounded cells; medulla: white; lower cortex: paraplectenchymatous with rounded cells; lower surface: white to brownish gray, often weakly rose colored; Apothecia: uncommon, up to 2 mm diam.; margins: partly sorediate; disc: dark brown to black, without pruina; ascospores: brown, 1-septate, intermediate between Physcia- and Pachysporaria-types, (16-) 17.5-21.5 (-22.5) x (7.5-) 8.5-10.5 (-12) µm; Pycnidia: usually numerous; conidia: subcylindrical, 4-6 x 1 µm; Spot tests: cortex K+ yellow, C-, KC-, P+ yellow; medulla K-, C-, KC-, P-; Secondary metabolite: upper cortex with atranorin.; Substrate and ecology: growing mainly on rocks, often manured by birds in open situations; World distribution: widely distributed but not common in northern and southern temperate regions; Sonoran distribution: one of the most common Physcias from the northern part of the Sonoran region and in scattered localities farther south, from coastal to upper montane habitats in Arizona, southern California, Baja California, Baja California Sur, Chihuahua and Sonora.; Notes: In its typical appearance P. tribacia is easily identified by its narrow lobes, shiny upper surface and paraplectenchymatous lower cortex. However, very small individuals might be difficult to distinguish from P. subtilis. In such cases the type of lower cortex separates them. Physcia halei is the fertile counterpart and, except for the lack of soralia, differs by having longer lobes. Material from North America determined as P. callosa in the sense of Thomson (1963) is included here.