Thallus: 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.