Thallus: foliose, loosely adnate, 6-15 cm in diam., irregularly lobate lobes: subirregular, elongate, plane to subconvex, separate and often forming an imbricate mat, 2-5 mm wide, not lobulate; apices: subrotund, 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: black, plane, shiny, sparsely rhizinate; rhizines: black, simple, 0.3-1 mm long Apothecia: common, substipitate, 2-15 mm wide, laminal on thallus; disc: cinnamon-brown to dark brown; margin: smooth, pruina absent asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, 9-11 x 5-7 µm Pycnidia: common, immersed conidia: bifusiform, 5-7 x 0.5 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla K+ yellow becoming dark red, C, KC-, P+ red Secondary metabolites: upper cortex with usnic acid (major); medulla with salazinic acid (major) and consalazinic acid (minor) and often with norstictic and protocetraric acids (both trace). Substrate and ecology: on acidic rocks, often in open or partially shaded habitats World distribution: Australasia, Africa, North and South America Sonoran distribution: occasional at intermediate elevations in Arizona, southern California, Baja California and Chihuahua. Notes: This widely distributed species is characterized by its large, conspicuous thalli with a jet black lower surface, the lack of maculae and vegetative propagules and the presence of salazinic acid in the medulla. Approximately the same range of morphological variation was observed in the Sonoran populations as in the Australasian ones. In the Sonoran region it could be confused with X. subtasmanica, the distinguishing features of which are described above under that species.