Thallus: foliose but sometimes appearing areolate centrally, tightly adnate, 3-8 cm in diam., irregularly lobate lobes: subirregular to sublinear, elongate, plane to subconvex, separate and contiguous, 0.8-2 mm wide, not lobulate; apices: subrotund, smooth to crenate, eciliate upper surface: yellow-green, sometimes darkening, smooth but becoming strongly rugose with age, shiny, epruinose and emaculate, without soralia, isidia, or pustulae medulla: white, with continuous algal layer lower surface: pale to medium brown, plane, moderately rhizinate; rhizines: pale brown, simple, 0.2-0.4 mm long Apothecia: common, adnate, 1-4 mm wide, laminal on thallus; disc: cinnamon-brown to dark brown; margin: smooth, pruina absent asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, 7-8 x 4-5 µm Pycnidia: common, immersed conidia: bifusiform, 5-6 x 0.5 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla K-, C-, KC-, P- Secondary metabolites: upper cortex with usnic acid (major), atranorin (major, minor or trace); medulla with dehydroconstipatic and subdecipienic acids (major), constipatic acid, protoconstipatic, protodehydroconstipatic and protosubdecipienic acids ( all minor). Substrate and ecology: on acidic rocks, often in open shaded habitats World distribution: Australasia, North and South America Sonoran distribution: occasional at lower to intermediate elevations in southern Arizona, Sonora and southern California. Notes: Xanthoparmelia oleosa is characterized by its small, tightly adnate, ±subcrustose thalli which often form areolae in older parts of the thallus, the brown lower surface and the presence of the constipatic acid chemosyndrome in the medulla. The chemistry of the fatty acid containing Xanthoparmeliae was not fully resolved by Hale (1990). Initially this species appeared to be fairly rare in northern Mexico and southern Arizona, but we now find that it is probably the most common fatty acid containing species.