Thallus: foliose, tightly adnate to adnate, 2-6 cm in diam., irregularly lobate lobes: subirregular to sublinear, short, plane to subconvex, separate, contiguous to somewhat imbricate, (0.5-)1-2 mm wide, not lobulate; apices: subrotund, smooth to crenate, eciliate upper surface: yellow-green, smooth, shiny, epruinose and emaculate, moderately isidiate isidia: initially globose, soon becoming cylindrical to irregularly inflated and branched, 0.1-0.2 mm in diam., 0.1-0.8 mm tall; tips: epicorticate, dull brown, occasionally erumpent; soralia and pustulae absent medulla: white, with continuous algal layer lower surface: pale brown, plane, moderately to densely rhizinate; rhizines: pale brown, simple, 0.2-0.5 mm long Apothecia: not observed Pycnidia: not observed Spot tests: upper cortex K-, C-, KC-, P-; medulla K+ yellow to orange, C-, KC-, P+ orange Secondary metabolites: upper cortex with usnic acid (major); medulla with norstictic acid (major) hyposalazinic acid (minor to trace), and and connorstictic acid (trace), and sometimes constipatic, dehydroprotoconstipatic, and protoconstipatic acids (all trace). Substrate and ecology: on acidic rocks, often in open, arid habitats World distribution: southwestern North America Sonoran distribution: frequent at lower elevations in Arizona, southern California, Baja California, Baja California Sur, Chihuahua and Sonora. Notes: Initially it appeared that X. maricopensis was restricted to the American Southwest whereas X. dierythra, another Xanthoparmelia with medullary norstictic acid major was only found in the upper Midwest. Although X. maricopensis is certainly more common in the Sonoran region, we have now found authentic X. dierythra in the region as well. In addition to the constant occurrence of hyposalazinic acid in X. maricopensis (not found in X. dierythra), X. maricopensis generally has narrower, more convex lobes.