Thallus: foliose, orbicular to irregular, 2-3 (-4) cm diam., often coalescing with other thalli forming colonies up to 15 cm diam., ± firmly adnate, repeatedly dichotomously lobate lobes: linear, elongate, narrow, c. (0.5-) 1-2 mm, flat to convex, end-lobes without soralia, slightly widening towards apices, minutely notched, eciliate upper surface: white to cream-colored or brownish to bluish gray, ± shiny, the lobe-tips sometimes darkening, very rarely pruinose, not ascending, sorediate soredia: abundant, farinose to ± granular, gray to bluish gray, in labiate soralia, on lateral lobes, sometimes semi-capitate, starting from lateral lobules upper cortex: prosoplectenchymatous medulla: white lower cortex: prosoplectenchymatous lower surface: pale to dark brown, rhizinate; rhizines: few, scattered, short and robust, sometimes irregularly branched, usually black Apothecia: rare (not seen in Sonoran material), substipitate, 3-8 mm in diam., margin: crenate or lacinulate; disc: brown to blackish brown asci: cylindrical to clavate, 8-spored ascospores: brown, l-septate, ellipsoid, 30-37 x 14-18 Pycnidia: not seen Spot tests: cortex K+ yellow C-, KC-, P+ yellow; medulla K+ yellow, C-, KC-, P- Secondary metabolites: cortex with atranorin and chloroatranorin; medulla with atranorin, zeorin and an unidentified triterpene. Substrate and ecology: growing on sunny, but moist rocks or on tree trunks in humid conditions World distribution: widely distributed in subtropical to temperate areas, extending to northern Scandinavia (Moberg & Holmåsen 1992) Sonoran distribution: mountains of SE Arizona and the Sierra Madre Occidental region of Chihuahua and Sonora. Notes: It is characterized by the striking contrast between the blue-gray soralia and the paler thallus and the presence of lower cortex. The lower surface of H. obscurata lacks a cortex and has a rusty orange pigment. Heterodermia albicans is much smaller and contains salazinic acid (K+ yellow-red).