Thallus: foliose, moderately to loosely adnate, (2-)4-20 cm in diam., irregularly lobate lobes: subirregular to sublinear, elongate, plane to subconvex, separate and contiguous, 0.8-3 mm wide, not lobulate; apices: subrotund to subtruncate, smooth to crenate, eciliate upper surface: yellow to yellow-green, smooth but becoming strongly rugose with age, dull to slightly 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, 0.1-0.3 mm long Apothecia: common, substipitate, 2-5 mm wide, laminal on thallus; disc: cinnamon-brown to dark brown; margin: smooth, pruina absent asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, 10-11 x 4-5 µm Pycnidia: common, immersed conidia: bifusiform, 5-6 x 1 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla K-, C-, KC-, P+ red Secondary metabolites: upper cortex with usnic acid (major); medulla with fumarprotocetraric acid (major), protocetraric acid (minor), confumarprotocetraric acid (minor), ±caperatic acid (major), ±norcaperatic acid (minor). Substrate and ecology: on acidic rocks, often in open or partially shaded habitats World distribution: montane areas of western North America extending south to Veracruz Sonoran distribution: common at intermediate elevations in Arizona, southern California, western Chihuahua, and Sonora. Notes: Mason Hale had a broader concept of X. novomexicana (Hale 1990), in which he included specimens with smaller, more tightly adnate, subcrustose thalli. We have now segregated the latter as X. tuberculata. X. novomexicana is distinguished from it by its larger thalli, (4-20 cm vs. 1-4 cm wide), the broader lobes (0.8-3 mm vs. 0.2-1 mm wide) and longer ascospores (10-11 µm vs. 8-9 µm long). Specimens previously labeled as X. pertinax from the Sonoran region all lack succinoprotocetraric acid, and thus are identical with X. novomexicana.