lobes: linear, elongate, narrow, 0.5-1 mm broad, up to 3 mm long, usually richly branched, weakly convex, widening, tips without soralia, not ascending, eciliate upper surface: gray to brownish gray, darker at lobe-tips, sometimes weakly pruinose, sorediate soredia: white to bluish gray, in soralia arising from small lateral knob-like structures forming small, ± continous marginal soralia towards the thallus center upper cortex: prosoplectenchymatous medulla: white lower cortex: prosoplectenchymatous, together with upper one occupying more than 2/3 of the thallus thickness lower surface: whitish to pale brownish, rarely dark gray, ± canaliculate; rhizines: weakly developed, usually short (c. 1 mm), pale to dark brown or black Apothecia: rare (not seen in Sonoran material), up to 2.5 mm in diam., margin becoming sorediate; disc: dark brown asci: clavate, 8-spored ascospores: brown, l-septate, narrowly ellipsoid, 21-32 x 7-14 µm Pycnidia: ± common, inserted with black weakly protruding tips conidia: bacilliform, 4-6 x 1 µm Spot tests: cortex K+ yellow C-, KC-, P+ yellow; medulla K+ yellow during deep red, C-, KC-, P+ orange Secondary metabolites: cortex with atranorin and chloroatranorin; medulla with atranorin and zeorin and salazinic acid (major) and ± unidentified terpenes. Substrate and ecology: growing on trees or rocks or over mosses on rocks in open situations World distribution: North and South America, the Azores, and East Africa Sonoran distribution: relatively common at intermediate elevations in SE Arizona, Chihuahua, and Sinaloa. Notes: Heterodermia albicans is characterized and distinguished by the narrow, convex, short lobes, corticate on the lower surface. The soralia arise from marginal knob-like structures forming small, ± continous marginal soralia towards the thallus center. Heterodermia speciosa also has a lower cortex but has larger, ± flat lobes and lacks salazininc acid. Heterodermia albicans differs from H. pseudospeciosa by more adpressed thallus, less delimited soralia and the absence of norstictic acid. TLC is recommended to separate the two species.