Thallus: orbicular or irregular, up to 4 cm diam., sometimes confluent with other thalli, loosely adnate lobes: radiating, truncate with open sinuses, up to 1 mm wide but mostly c. 0.5 mm, eciliate upper surface: whitish gray to cream-colored (margin often paler), mostly without pruina and maculation; soredia and isidia absent upper cortex: paraplectenchymatous medulla: white lower cortex: prosoplectenchymatous, cells sometimes short and thick-walled lower surface: white to brownish, with few pale rhizines Apothecia: usually abundant, variable in size, up to 1.5 mm diam., but usually less than 1 mm; margins: thick; disc: often pruinose ascospores: brown, 1-septate, Physcia- or Pachysporaria-type, (14-) 17.5-22.5 (-28) x (6.5-) 8-10 (-12) µm Pycnidia: ± abundant 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 on trees in open situations World distribution: widely distributed species within the arctic, boreal and temperate vegetation zones Sonoran distribution: a relatively frequent species on trees from low to upper montane localities of Arizona, southern California, Baja California, Baja California Sur, and Sonora. Notes: The abundant apothecia, the K medulla and corticolous habitat are the main characters of P. stellaris. There is a slight difference in morphology and appearance within the range of its distribution (Moberg 1986: 860) and the Sonoran Desert material is closer to the southern type as would be expected. The closely related species P. convexella differs in lobe shape and substrate preference.