Thallus: foliose, up to 5 cm in diam., irregular and sometimes pulvinate, less often lobes: linear to more often somewhat irregular-rounded and partly imbricate, 1-2 mm broad, mostly flat to weakly concave or convex, ± prostrate, frequently with scattered marginal lobules upper surface: gray to gray-brown or commonly brown to dark reddish brown, orbicular, with a partial (lobe ends) or sometimes almost completely pruinose, sorediate soredia: granular to strongly isidioid in older parts, darkening with age, in labriform soralia, primarily on the upturned lobe ends of lateral lobes or sometimes on main lobes upper cortex: scleroplectenchymatous medulla: white lower cortex: absent or weakly organized and ± prosoplectenchymatous lower surface: pale throughout (small specimens) or darkening centrally (in larger, better developed specimens), the lobe ends distinctly whitish and ecorticate, usually with distinctive blackish striations developing and becoming more numerous inward, these eventually coalescing to form a poorly organized and very dull 'cortex' in many (but not all) specimens; rhizines: black and squarrosely branched Apothecia: rare, up to 1.5 mm in diam., the margin entire, sometimes becoming lobulate (lobules sometimes with labriform soralia) ascospores: 26-32 (-34) x 15.5-18 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: bark, rock, mosses over rock World distribution: North America, Europe, Asia, Africa and South America Sonoran distribution: southern California and Arizona (primarily upper elevation areas in the mountains). Notes: The distinctive terminal labriform soralia (occasionally very sparse) and the partly to mostly ecorticate lower surface, usually with blackish striations, distinguish this species from all others in the study area. The lack of positive spot tests in both the medulla and the soralia also serve to separate it from other species with labriform or near labriform soralia (e.g. P. leucoleiptes).