Thallus: foliose, adnate to tightly adnate, 4-7 cm in diam., irregularly lobate lobes: subirregular to sublinear, elongate, plane, separate, continguous or imbricate, 0.3-1 mm wide, sometimes becoming lobulate lobules: sublinear, sparingly dichotomously branched, 0.1-0.2 mm wide; apices: subtruncate, smooth to crenate, eciliate upper surface: yellow-green or darkening with age, smooth, shiny at first, then dull, ±cracked with age and becoming areolate; epruinose and emaculate, without isidia, soralia or pustulae medulla: white, with continuous algal layer lower surface: pale brown or brown, plane, moderately rhizinate; rhizines: pale brown to black-brown, simple, 0.1-0.4 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, 8-10 x 4-5 µm Pycnidia: common, immersed conidia: bifusiform, 5-7 x 0.5 µm Spot tests: upper cortex K+ yellow to orange, C-, KC-, P+ orange; medulla K+ yellow becoming dark red, C-, KC-, P+ orange Secondary metabolites: upper cortex with usnic acid (major), atranorin (trace); medulla with stictic acid (major), constictic and norstictic acids (both minor) and cryptostictic and peristictic acids (both trace), ±menegazziaic acid (minor/trace). Substrate and ecology: on acidic rocks World distribution: southwestern North America Sonoran distribution: Arizona. Baja California, Sonora and Chihuahua at intermediate elevations. Notes: Like the well-known X. cumberlandia, this new species is characterized by the lack of vegetative propagules, the pale lower surface and the presence of the stictic acid chemosyndrome in the medulla. However, X. subcumberlandia can be distinguished by the smaller, more tightly adnate thallus (4-7 cm vs. 6-12 cm wide), that often becomes areolate in the center, and the narrower lobes (0.31 mm vs. 1-2 mm wide). On the advise of M.E. Hale this taxon was initially identified as X. arseneana (Gyeln.) Hale (Nash 1974), but Hale (1990) subsequently demonstrated that the type of that species had fumarprotocetraric acid. Consequently, a new name is required for this species.