Thallus: foliose, adnate to loosely adnate, 10-20 cm in diam., irregularly lobate lobes: sublinear, elongate, plane to subconvex, separate and densely imbricate, 0.5-1.5 mm wide; becoming lobulate centrally; lobules: 0.4-0.8 mm wide, subascending black-rimmed; apices: subrotund, smooth to crenate, eciliate upper surface: dark yellow-green, smooth, shiny, epruinose and emaculate, without soralia, isidia, or pustulae medulla: white, with continuous algal layer lower surface: brown, rugose towards tips, sparsely to densely rhizinate; rhizines: pale brown, simple to furcated, 0.3-0.8 mm long Apothecia: common, sub-stipitate, 2-6 mm wide, laminal on thallus; disc: cinnamon-brown to dark brown; margin:smooth, pruina absent asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, 9-10 x 5-6 µm Pycnidia: common, immersed conidia: bifusiform, 5-6 x 1 µm Spot tests: upper cortex K-, C, KC-, P-; medulla K+ yellow to dark red, C-, KC-, P+ orange Secondary metabolites: upper cortex with usnic acid (major); medulla with salazinic acid (major) and consalazinic acid (minor), and sometimes norstictic and protocetraric acids (trace). Substrate and ecology: on acidic rocks, often in semi-open, woodland habitats World distribution: throughout Australasia and in southwestern North America with extentions to central Mexico Sonoran distribution: occasional at intermediate elevations in Arizona, Baja California and western Chihuahua. Notes: In well developed specimens, the dense lobulae give an overall impression of X. digitiformis forming a mat. Its primary lobes are more loosely adnate and much narrower (0.5-1.5 mm vs. 1.5-4 mm) than those in either the South American X. taractica or the North American X. coloradoënsis. The latter species is more adnate and has much more irregularly developed lobulae in comparison with X. digitiformis.