Thallus: subfruticose to fruticose, forming small cushions up to 1.5 cm wide or extensive colonies, lobate lobes: dorsiventral, raised, 0.1-0.6 mm wide; tips: ±terete, nodulose, 0.05-0.2 mm wide upper surface: gray or (often partly) yellow to orange, smooth, sorediate soredia: granular, blastidious, marginal and at tips and margins of extremely narrow branches medulla: white, reticulate, with short hyphae lower surface: white, smooth, very rarely with short, white hapters Apothecia: common but scattered, laminal, sessile, up to 2 mm in diam.; margin: smooth, sometimes with blastidia; disc: orange; epihymenium: brown, c. 10 µm thick; hymenium: hyaline below, 35-55 µm tall; hypothecium: hyaline to pale brown, 10-80 µm thick; paraphyses: simple or somewhat branched, cylindrical, septate asci: clavate, 8-spored ascospores: ellipsoid, polarilocular, hyaline, 11-16 x 4.5-7 µm; septum: 2-5 µm wide Pycnidia: common, immersed, orange even when thallus surface is gray conidia: ellipsoid, 2.5-3.5 x 1-1.5 µm Spot tests: upper surface K+ purple, C-, KC-, P- Secondary metabolites: parietin (major), fallacinal, emodin, teloschistin (major) and parietinic acid. Habitat and ecology: on dry twigs, rarely bark (two collections on palms), once collected on volcanic rock, in dry microclimate influenced by coastal fog World and Sonoran distribution: Baja California and southern California (San Nicolas Island). Notes: This recently described species is similar to X. candelaria, but the lobes of X. tenuiloba are nodulose, more slender and narrow, and mostly not as densely crowded. The pigmentation of the two taxa differ: X. tenuiloba is mostly gray or dull yellow, and only very rarely as brightly pigmented as X. candelaria. Xanthoria tenuiloba seems to be confined to dry twigs of shrubs in coastal habitats.