Thallus: foliose, adnate, 3-10 cm in diam., irregularly lobate lobes: subirregular, elongate, plane to subconvex, separate, contiguous to somewhat imbricate, (1-)2-3 mm wide, not lobulate; apices: subrotund, smooth to crenate, eciliate upper surface: yellow-green, smooth, shiny, epruinose and emaculate, moderately to densely isidiate isidia: abundant, initially globose, soon becoming cylindrical, ±coralloid branched to irregularly inflated, 0.1-0.2 mm in diam., 0.1-0.8 mm tall; tips: epicorticate, dull brown, sometimes weakly erumpent; soralia and pustulae absent medulla: white, with continuous algal layer lower surface: dark brown to black, plane, moderately to densely rhizinate; rhizines: black, simple, 0.2-0.5 mm long Apothecia: rare, substipitate, 2-4 mm wide, laminal on thallus; disc: cinnamon-brown to dark brown; margin: smooth, pruina absent asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, 9-12 x 5-6 µm Pycnidia: rare, immersed conidia: bifusiform, 5-6 x 1 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla K-, C-, KC-, P- Secondary metabolites: upper cortex with usnic acid (major); medulla with hypoprotocetraric acid (major), 4O-demethylnotatic acid (submajor), conhypoprotocetraric acid (minor). Substrate and ecology: on acidic rocks, rarely on wood, usually in open, arid habitats World distribution: southwestern North America Sonoran distribution: common at lower elevations in Arizona and Sonora. Notes: Xanthoparmelia nigroweberi is distinguished by the adnate, foliose thallus with an isidiate upper surface, the dark brown to black lower surface and the presence of hypoprotocetraric and 4-O-demethylnotatic acids in the medulla. Xanthoparmelia weberi has identical medullary chemistry and superficially similar thalli and isidia but is distinguished in having a pale tan to brown lower surface. Xanthoparmelia nigroweberi also comes close to X. patula (Brusse) Elix from South Africa. However, in X. patula the lower surface is jet-black throughout (dark brown to black in X. nigroweberi) and the isidia are simple and globose (cylindrical and ±coralloid branched in X. nigroweberi).