Thallus: foliose, loosely adnate, 3-7 cm in diam., subirregularly lobate lobes: sublinear, elongate, plane to subconvex, separate and often forming an imbricate mat, 1-2 mm wide, lobulate-laciniate in thallus center, laciniae sublinear-elongate, subdichotomously branched, ±divaricate and constricted, 0.3-0.8 mm wide; apices: incised, smooth to crenate, eciliate upper surface: yellow to yellow-green, smooth but becoming strongly rugulose with age, dull to 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, often grouped subapically 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, 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 (in trace amounts). Substrate and ecology: on acidic rocks, often in open or partially shaded habitats World distribution: western North America Sonoran distribution: occasional at intermediate elevations in Arizona and Chihuahua. Notes: Like the well known, cosmopolitan X. tasmanica, this new species is characterized by the lack of vegetative propagules, its black lower surface with sparse rhizines and the presence of the salazinic acid chemosyndrome in the medulla. However, X. subtasmanica can be distinguished by its smaller thallus (3-7 cm vs. 6-15 cm wide) and its narrower lobes (1-2 mm vs. 2-5 mm wide) which develop prominent lobulate-laciniae in the thallus center. These characteristic laciniae are sublinear-elongate, dichotomously branched and often divaricate and constricted.