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DescriptionThallus: orbicular to irregular, to 5 cm diam.; lobes: up to 3 mm, usually narrower, sometimes overlapping, sometimes distinctly separated, eciliate; upper surface: gray to dark gray with white maculae; soredia: in marginal to ± laminal, often capitate soralia; upper cortex: paraplectenchymatous; medulla: white; lower cortex: prosoplectenchymatous with the lowermost part gradually forming rounded, isodiametric cells; lower surface: white to dark gray with dark, sometimes with black rhizines; Apothecia: not seen in the Sonoran Desert material, elsewhere common to rare, sessile, up to 1.5 mm wide, margin: entire or crenulate; disc: brown-black, sometimes pruinose; hymenium: hyaline below, brownish above; hypothecium: hyaline or yellowish; paraphyses: simple or rarely branched, capitate; ascospores: brown, 1-septate, Physcia-type, 17-24 x 7-11 µm; Pycnidia: not seen in the Sonoran Desert material, elsewhere rare; conidia: subcylindrical, 4-6 x 1 µm; Spot tests: upper cortex and medulla K+ yellow, C-, KC-, P+ yellow; Secondary metabolites: upper cortex with atranorin; medulla with atranorin, zeorin and sometimes unidentified triterpenes.; Substrate and ecology: mainly growing on acid rocks in ± open situations, but it is also known as corticolous (not in the Sonoran Desert); World distribution: widely distributed within the arctic, boreal and temperate vegetation zones; Sonoran distribution: one of the most common Physcias on rocks at coastal to high elevations in the Sonoran region from Arizona, southern California, Baja California, Baja California Sur and Sonora.; Notes: Physcia caesia may sometimes be difficult to distinguish from P. poncinsii but the characteristic crateriform soralia and the distinctly convex lobes of the latter make them easy to separate.