Thallus: foliose, up to 11 cm in diam., somewhat orbicular or often irregular and sometimes entangled with other thalli lobes: irregular-flabellate, contiguous or imbricate, mostly 2-4 mm broad, usually ascending on the ends and therefore distinctly concave upper surface: gray-brown to dark brown, usually whitish pruinose essentially throughout, sorediate-isidiate sorediate isidia: arising first on the lobe margins and under up-turned lobes as very granular soredia, and also later in numerous small patches on the upper surface, propagules becoming rather irregular and strongly isidioid (somewhat similar to blastidia?), often coalescing in central parts of mature thalli upper cortex: paraplectenchymatous medulla: mostly white to off-white but occasionally pale yellowish in patches or where exposed lower cortex: irregularly prosoplectenchymatous lower surface: darkening and becoming black inwardly, the peripheral lobes pale tan to whitish on the ends, dull to weakly shiny; rhizines: black and squarrosely branched Apothecia: not seen (likewise for pycnidia) Spot tests: cortex all negative; medulla usually K-, C-, KC-, P-, but sometimes K+ pale yellow and KC+ yellow to orange in scattered areas; soralia (and associated medulla) K-, C-, KC-, P-, or some places K+ and KC+ yellowish Secondary metabolites: variolaric acid, also sometimes small amounts of secalonic acid A. Substrate and ecology: rock or mosses over rock, rare on bark World distribution: western North America: Utah, Colorado, Arizona and California Sonoran distribution: higher elevations in Arizona, southern California and Baja California Sur. Notes: Smaller individuals of this species might be confused with either P. enteroxantha or P. isidiigera, depending on whether or not the sometimes positive medullary/soredial spot tests are observed. Normally, the large size of the thallus and broad peripheral lobes, their tendency to be concave and ascending, and the distinctive laminal soredia which become isidioid, should be enough to distinguish it from those species. The usual presence of variolaric acid also distinguishes P. isidiomuscigena from both those species.