Thallus: orbicular to irregular, loosely attached, up to 5 cm diam. lobes: flat to concave, broad, up to 4 mm wide with rounded tips; margins: often prominent because of the white rim, eciliate upper surface: gray to bluish gray, pruinose, usually with a white "frosty" rim along the lobe margins; soredia and isidia absent upper cortex: paraplectenchymatous medulla: white lower cortex: prosoplectenchymatous to labyrinthiform with thick walls, usually well delimited lower surface: white to pale brownish, with a few concolorous rhizines Apothecia: usually abundant, shortly stalked, up to 5 mm diam.; margin: prominent, crenulate ascospores: brown, 1-septate, Pachysporaria-type, (17-) 18.5-22 (-22.5) x (8.5-) 9-10.5 (-11) µm Pycnidia: abundant, immersed, appearing as black warts on the surface conidia: subcylindrical, 4-6 x 1 µm Spot tests: cortex K+ yellow, C-, KC-, P+ yellow; medulla only weakly K+ yellow, C-, KC-, P- Secondary metabolites: upper cortex with atranorin; medulla with triterpenes (not zeorin). Substrate and ecology: growing on branches and twigs of various trees and shrubs in open situations World and Sonoran distribution: so far known only from a few scattered coastal localities in Sonora and Sinaloa. Notes: Physcia tretiachii is a very distinct species recognized by the broad lobes with a white marginal rim. No species seems to be very closely related, but it has some similarities to P. atrostriata Moberg which, however, has a dark, prosoplectenchymatous lower cortex and a different chemistry (zeorin).