Thallus: foliose, adnate to loosely adnate, 4-10 cm in diam., irregularly lobate lobes: subirregular, elongate, plane to subconvex, separate, contiguous to somewhat imbricate, (0.5-)1.5-4 mm wide, not lobulate; apices: subrotund, smooth to crenate, eciliate upper surface: yellow-green, smooth, shiny, epruinose and emaculate, moderately to densely isidiate isidia: initially subglobose, soon becoming cylindrical to coralloid branched, 0.1-0.2 mm in diam., 0.1-0.5 mm tall; tips: syncorticate, brown to blackish brown, sometimes weakly erumpent; soralia and pustulae absent medulla: white, with continuous algal layer lower surface: pale to medium brown, plane, moderately to densely rhizinate; rhizines: pale brown, simple, 0.2-0.5 mm long Apothecia: rare, substipitate, 2-7 mm wide, laminal on thallus; disc: cinnamon-brown to dark brown; margin: smooth, pruina absent asci: clavate, 8spored ascospores: hyaline, simple, ellipsoid, 9-10 x 4-5 µm Pycnidia: rare, immersed conidia: bifusiform, 6-7 x 1 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla K+ yellow to dark red, C-, KC-, P+ orange Secondary metabolites: upper cortex with usnic acid (major); medulla with salazinic acid (major) consalazinic acid (minor), and usually norstictic and protocetraric acids (trace). Substrate and ecology: on acidic rocks, often on soil near the coast, in open, arid habitats World distribution: Africa, Asia, Australasia, southern North America and South America Sonoran distribution: common at lower elevations in Arizona, southern California, Baja California, Baja California Sur, Chihuahua and Sonora. Note: Xanthoparmelia mexicana is probably the most common Xanthoparmelia in the Sonoran region. Typically it is a somewhat broad lobed species with dense isidia, a pale to reddish brown lower cortex and salazinic acid as the major medullary metabolite. In the interior deserts of central Arizona, X. mexicana often has narrower lobes and a higher degree of adnation than coastal populations.