Thallus: foliose, forming small cushions up to 3 cm wide or larger colonies, tightly adnate, lobate lobes: dorsiventral, flat to convex, 0.2-0.7 mm wide; tips: 0.1-0.3 mm wide, ±rotund upper surface: yellow to bright orange, smooth to somewhat shiny, esorediate medulla: white, reticulate, with short hyphae lower surface: white, somewhat wrinkled, with scattered short, white hapters Apothecia: extremely abundant (to crowded), laminal, stipitate, up to 4.5 mm in diam.; margin: smooth but sometimes crenulate; disc: orange; epihymenium: brown, 10 µm thick; hymenium: hyaline below, c. 40-90 µm tall; hypothecium: hyaline to pale brown, 15-50 µm thick; paraphyses: simple or slightly branched, cylindrical, septate asci: clavate, 8spored ascospores: ellipsoid, polarilocular, hyaline, 11-15 x 5-8 µm; septum: 2-6.5 µm wide Pycnidia: common, immersed, concolorous with upper surface or slightly darker conidia: ellipsoid, 2-3 x 1-1.5 µm Spot tests: upper surface K+ purple, C-, KC-, P- Secondary metabolites: parietin (major), fallacinal, emodin, teloschistin and parietinic acid. Habitat and ecology: on bark, mostly twigs, often in humid microclimates World distribution: circumpolar and extending into temperate regions, and Australasia(?) Sonoran distribution: coastal regions of southern California and Baja California. Notes: Xanthoria polycarpa is characterized by its +cushion-like thallus, with short and narrow, convex lobes, frequently almost completely covered with apothecia. The ellipsoid conidia as well as the morphology of the lobes and holdfasts (hapters vs. rhizines) are other important characters separating X. polycarpa from Xanthomendoza hasseana and X. montana. Xanthoria tenax has a flat, tightly adnate thallus with broader lobes which are attached by the lower surface almost to the lobe tips. Lindblom (1997) considered X. ramulosa synonymous to X. polycarpa, but the taxonomy is in need of further investigation.