Thallus: foliose, orbicular, up to 5 cm diam. or irregular and confluent with other thalli, ± closely adnate, dichotomously to subirregularly lobate lobes: linear, ± elongated, 0.5-1.5 mm broad, with ± lobulate margins, eciliate upper surface: whitish gray, gray to dark gray often with darker margins and tips, sometimes with a weak pruina; soredia. and isidia absent upper cortex: prosoplectenchymatous medulla: white lower cortex: prosoplectenchymatous lower surface: white to pale gray or brownish, rhizinate; rhizines: abundant, pale gray to black, sparsely branched Apothecia: common, up to 5 mm diam. with abundant lobules along margins on older apothecia; disc: brown to blackish brown, epruinose asci: subclavate to clavate, 8-spored ascospores: brown, l-septate, Pachysporaria-type, (25-) 27-32 (-37.5) x (12-) 13-17 (-19.5) µm Pycnidia: ± abundant conidia: bacilliform, 4-5 x 1µm Spot tests: cortex K+ yellow, C-, KC-, P+ yellow; medulla K+ yellow to red, C-, KC-, P+ orange Secondary metabolites: cortex with atranorin and chloroatranorin; medulla with atranorin, zeorin and salazinic acid (major). Substrate and ecology: on bark of various trees, rarely on rock World distribution: has a limited distribution in SE Arizona, Mexico and Central America Sonoran distribution: mountains of SE Arizona, adjacent Sierra Madre Occidental regions of Chihuahua and Sonora and the mountains of southern Baja California Sur. Notes: Heterodermia tropica is very similar to H. diademata and the major difference is presence of salazinic acid. The relation between H. tropica and H. albicans is probably the same as the relation between H. diademata and H. speciosa.