Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: foliose, ± firmly appressed, up to 5 cm in diam. lobes: flat, slightly convex or concave towards the periphery, 0.8-2.0 mm wide upper surface: gray brownish gray or almost white; pseudocyphellae: sparse to abundant, usually restricted to the peripheral parts of the lobes, initially marginal, then spreading laminally, sometimes reticulately confluent; pruina: punctiform, rarely confluent polysidiangia: developing from marginal, later also from laminal pseudocyphellae, initially distinctly corticate, later bursting open and simulating nodular soralia; true soralia and isidia lacking medulla: white throughout lower surface: black in center, paler towards lobe tips; rhizines: ± dense, blackish, furcately divided Apothecia: obscurascens-type, rare, laminal, 0.6-2.0 mm wide; exciple: outer part white pruinose (or rarely some apothecia without pruina); disc: black, not pruinose internal stipe: distinct, white throughout ascospores: one-septate, brown, 17-21 x 5.5-7 µm Pycnidia: immersed conidia: bacilliform, 3-4 x 1 µm Spot tests: upper cortex K+ yellow, C-, KC-, P+ yellow; medulla upper and lower part K-, C-, KC-, P+ orange red; internal stipe upper and lower part K-, C-, KC-, P+ orange red Secondary metabolites: upper cortex with atranorin; medulla with terpenes of a characteristic array on TLC plates, testacein. Substrate and ecology: on bark and acidic rocks in montane areas from 1000-2200 m World distribution: probably a Sonoran endemic Sonoran distribution: southern mountains of Baja California Sur and the Sierra Madre Occidental region of Chihuahua. Notes: Previous collections of this species have been reported under the name P. retirugella.