Thallus: foliose, tightly adnate, 7-10 cm in diam., irregularly lobate lobes: sublinear, elongate, plane, separate, contiguous to barely imbricate, 1.5-4 mm wide, rarely becoming lobulate (lobulae c. 1 mm wide); apices: sub-rotund, smooth to crenate, eciliate upper surface: yellow-green and darkening with age, smooth but becoming rugulose and cracked with age, dull but shiny apically, epruinose and emaculate, without isidia, soralia or pustulae medulla: white, with continuous algal layer lower surface: pale tan to brown, plane, sparsely to moderately rhizinate; rhizines: brown to dark brown, simple, 0.3-0.5 mm long Apothecia: common, substipitate, 1-2 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 Ám Pycnidia: common, immersed conidia: bifusiform, 5-6 x 0.5 Ám Spot tests: upper cortex K-, C-, KC-, P-; medulla K-, C-, KC+ yellow, P- Secondary metabolites: upper cortex with usnic acid (major); medulla with barbatic acid (major), 3-α-hydroxybarbatic acids (major or minor), 4-O-demethylbarbatic acid (major, minor or trace), and squamatic acids (minor or trace), and baeomycesic acid (trace). Substrate and ecology: on acidic rocks, often in open, arid and woodland habitats World and Sonoran distribution: locally common at intermediate elevations in the Sierra Madre Occidental of Chihuahua and adjacent Texas. Notes: Xanthoparmelia eganii was previously reported from North America as X. barbatica (Elix) Egan (Egan 1982), because both have similar overall morphology and chemistry. Xanthoparmelia barbatica, however, is clearly distinguished from X. eganii by its being more loosely adnate, its more regular, sublinear lobes, its common development of extensive narrower (0.7-1.5 mm wide), sublinear-elongate lobules, its shorter spores (7-9 Ám vs. 9-12 Ám) and its minor, but consistent chemical differences. Whereas X. barbatica invariably contains dehydroconstipatic acid (major) and traces of constipatic acid, these fatty acids do not occur in X. eganii. Furthermore, 3-α-hydroxybarbatic acid, baeomycesic acid and squamatic acid which are present in X. eganii have not been observed in X. barbatica (Elix and Nash 1999).