Thallus: foliose to subcrustose, forming small rosettes up to 2.5 cm wide, tightly adnate, lobate lobes: dorsiventral, XANTHORIA flattened, 0.2-0.7 mm wide; tips: 0.2-0.6 mm wide, rotund to fan-shaped upper surface: yellow to orange, smooth, slightly to heavily pruinose, esorediate medulla: white, reticulate, with short to elongate hyphae lower surface: attached to the substrate, with a lower cortex present below lobe tips only, near lobe tips white, smooth, rarely with short, white hapters Apothecia: common, laminal, sessile, up to 3 mm in diam.; margin: smooth to crenulate; disc: orange, sometimes slightly pruinose; epihymenium: brown, c. 10 µm thick; hymenium: hyaline below, 60-95 µm tall; hypothecium: hyaline to pale brown, 25-100 µm thick; paraphyses: simple or slightly branched, cylindrical, septate asci: clavate, 8-spored ascospores: narrowly oblong, polarilocular, hyaline, 11-17 x 5-7.5 µm; septum: 2.56 µm wide Pycnidia: common, immersed, slightly darker than the upper surface conidia: ellipsoid, 2.5-3.2 x 1-1.5 µm Spot tests: upper surface K+ purple, C-, KC-, P- Secondary metabolites: parietin (major), fallacinal (+major), emodin, teloschistin (+major) and parietinic acid. Habitat and ecology: on bark, often on twigs, often in dry microclimates influenced by coastal fog World distribution: North America Sonoran distribution: coastal regions of Baja California, Baja California Sur and southern California, Sonora and Sinaloa. Notes: Xanthoria tenax is a very common species characterized by the tightly adnate, apotheciate, often lightly pruinose thallus with rounded to fan-shaped lobe tips, which are the only parts of the thallus having a lower cortex. Xanthomendoza concinna and Xanthoria polycarpa are similar to X. tenax in being small and apotheciate. In addition, X. concinna normally develops a pruina. The main differences are that the former two species form cushion-like thalli with a distinct lower cortex, while X. tenax is flat and tightly adnate to the substrate. Furthermore, X. concinna differs by having bacilliform conidia and apothecia that often appear somewhat inflated.