Thallus: crustose, endolithic or effuse, up to 8 cm in diam., thin, continuous, rimose to areolate or scurfy, occasionally with a black to brown prothallus evident marginally or between areoles areoles: irregularly shaped, 0.4-0.5(-0.8) mm in diam. surface: white, yellowish white, greenish gray or pale gray, rarely tinged orange, rough, lacking asexual propagules cortex: hyaline or occasionally encrusted with orange crystals above, 10-35 µm thick, usually lacking an epinecral layer medulla: white, I-, with hyphae 3.5-4 µm in diam.; algal layer: 60-65 µm thick with cells 9-10(-14) in diam Apothecia: +round, sometimes clustered, sessile, constricted at the base, 0.3-1(-1.5) mm in diam., disc: black, plane to convex, dull, occasionally pruinose margin: initially distinct, +persistent, black or occasionally rusty red, usually entire exciple: black to dark brown peripherally, brown to dark brown internally, 60-100 µm wide above, 80-120 µm wide below, with hyphae c. 3 µm wide hymenium: hyaline below, dark brown or olivaceous brown above, I+ blue, 60-90(-120) µm tall; paraphyses: branched and anastomosing, 1.2-1.8 µm wide below, 2-3.5 µm wide apically; hypothecium: pale brown to usually dark brown, up to 150-270 µm thick asci: clavate, 55-80 x 10-20 µm, 8-spored ascospores: hyaline, simple, ellipsoid, 10-17(-22) x 5-9(-9.5) µm, halonate Pycnidia: immersed; conidia: hyaline, bacilliform, 9-14 x c. 1 µm Spot tests: thallus K- or +yellow, C-, KC-, P- or +orange Secondary metabolites: +stictic acid (major), +constictic, +norstictic and +cryptostictic acids (all minor or trace), or none detected. Substrate and ecology: on siliceous rocks and pebbles at higher elevations, very rarely on wood or bark World distribution: throughout temperate and arctic alpine areas in Europe, Asia, North and South America and Australasia Sonoran distribution: in the higher mountains of central and eastern Arizona. Notes: Elsewhere a chemotype of P. crustulata with norstictic acid (major) is known. The somewhat more robust P. macrocarpa usually occurs at higher elevations and often lacks secondary metabolites. In addition, the spores of P. macrocarpa are usually larger than those of P. crustulata.