Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: foliose, up to 7 (rarely 8 or 9) cm in diam., irregular to more often orbicular lobes: linear-elongate and discrete to somewhat irregularly rounded and partly imbricate, 1-2 (-3) mm broad, usually ± flat to irregularly concave, prostrate upper surface: gray to mostly gray-brown or brown, with a partial (lobe ends) or occasionally almost completely pruinose, sorediate soredia: in primarily marginal soralia which are elongate (usually becoming ± continuous except on the peripheral lobes), not or only partly reflexed, only rarely appearing weakly labriform, some laminal soralia sometimes also developing, especially in older thallus parts; individual soredia granular to pseudocorticate and isidioid upper cortex: scleroplectenchymatous medulla: white to off-white lower cortex: irregularly prosoplectenchymatous lower surface: peripheral lobes usually dark tan-brown, occasionally paler but almost never white, darkening to dark brown or black inwardly, dull or weakly shiny; rhizines: black and squarrosely branched Apothecia: infrequent, up to 3 mm in diam., sessile, the margin thick and entire, not usually lobulate, soon becoming sorediate ascospores: 26-32 (-35) x 13-18 (-20) µm Spot tests: cortex, medulla and soralia normally with all negative, very rarely the soralia K+ and/or KC+ slight yellow Secondary metabolites: usually none detected, variolaric acid very rarely accessory (apparently in the soralia). Substrate and ecology: bark, rock, or over mosses on rock (in the study area, known so far only from mosses over rock) World distribution: North America, Europe, Asia Sonoran distribution: central and eastern Arizona in the mountains. Notes: This species is distinguished by the linear marginal soralia and the usual lack of positive spot tests. A similar species occurring primarily in the western part of the Sonoran Region is P. isidiigera, which is distinguished by having a paraplectenchymatous upper cortex. Both these species must also be carefully distinguished from P. enteroxantha, which can usually be identified by the positive K and KC (yellowish) spot tests in the medulla (and often also on the soralia).