Thallus: small foliose, up to 1 cm wide or coalescing to form extensive colonies, tightly adnate to adnate, lobate lobes: dorsiventral to subterete, loosely imbricate, narrow, 0.1-0.5 (-1.2) mm wide upper surface: lemon yellow to mustard yellow, paling to yellow green in shade, smooth to somewhat coarse, sorediate soredia: granular, blastidious, marginal to submarginal or at lobe tips upper cortex: c. 10-30 µm thick medulla: white, very thin lower cortex: c. 20 µm thick lower surface: white to pinkish, somewhat shiny; rhizines: simple, white Apothecia: rare, laminal or appearing terminal, sessile, up to c. 1 mm diam.; margin: smooth but often sorediate or lobulate; disc: darker yellow than the thallus; epithecium: c. 5-10 µm thick; hymenium: c. 60-90 µm tall; paraphyses: simple, cylindrical to submoniliform with up to 5 µm wide tips; hypothecium: c. 50 µm thick asci: clavate, >30-spored ascospores: uni- or biguttulate, colorless, 7-10 x 4-6 µm Pycnidia: common, appearing as raised warts on the thallus surface, concolorous with upper surface or darker conidia: ellipsoid to narrowly ellipsoid, 2-3 x 1,5 µm Spot tests: upper surface K- (or + deeper yellow), C-, KC-, P-; medulla K-, C-, KC-, P- Secondary metabolites: calycin (major) and pulvinic dilactone (minor). Habitat and ecology: on rock or bark, often in relatively eutrophicated habitats World distribution: pan-arctic and southwards into north temperate regions, extending farther south in maritime areas; also occurring in subantarctic regions and Australasia Sonoran distribution: one of the most common of small foliose lichens throughout the region from coastal regions to at least 3000 m in Arizona, southern California, Chihuahua, Sinaloa, Sonora, Baja California and Baja California Sur. Notes: In var. effusa the thallus is very rudimentary. Most material collected in California and previously identified as C. concolor belongs to Candelaria sp.