Thallus: foliose, up to 7 cm in diam., irregular to more often orbicular lobes: linear to somewhat irregular-rounded and partly imbricate, 1-2 (-3) mm broad, ± flat to weakly concave, prostrate upper surface: gray to brown, with a partial (lobe ends) or often almost complete pruina, sorediate soredia: in marginal soralia which are elongate and ± continuous in older parts, laminal soralia sometimes developing in older regions; individual soredia granular to pseudocorticate and isidioid or at times becoming coralloid-stacked and resembling true isidia, pale but darkening with age upper cortex: paraplectenchymatous medulla: white lower cortex: irregularly prosoplectenchymatous lower surface: peripheral lobes pale tan to almost white (sometimes up to 5 mm from tip), inwards becoming dark brown or usually black; rhizines: blackening, squarrosely branched Apothecia: infrequent but not rare, up to 2 mm in diam., the margin thick and entire, becoming sorediate or isidioid but not usually lobulate ascospores: 28-38 x 15.5-18 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: bark or wood, less often on rock World distribution: western North America Sonoran distribution: southern California and Baja California. Notes: In some of its forms, this species is most similar to P. detersa, which in western North America appears to be restricted to the southern Rocky Mountains, usually on mosses over rock. That species can also be distinguished from P. isidiigera by its scleroplectenchymatous upper cortex. Specimens of P. enteroxantha with very low concentrations of secalonic acid A in the medulla and/or soralia are likely to be identified as this species if the medullary spot tests are not very carefully carried out and interpreted.