Description: Thallus minutely foliose to subsquamulose, bright lemon yellow, paling to yellow-green in shade, forming up to 5 mm wide rosettes which often coalesce into up to 30 cm wide colonies. Lobes narrow and short, subsquamulose, 0.1-0.6 mm wide, adnate or ascending, dorsiventral, sparse to imbricate, with granular, 35-40 μm wide blastidia most common on the more or less fan-shaped lobe-ends. Upper cortex c. 10-45 µm thick; medulla white, very thin; lower cortex absent. Lower surface with an arachnoid appearance, white and transparent, showing the green algal layer. Apothecia extremely rare in Europe, sessile, to c. 1 mm diam., with a yellow, plane to finally convex disc, and a smooth but often sorediate margin. Epithecium yellow-brown, granular; hymenium colourless; paraphyses simple, cylindrical to submoniliform; hypothecium colourless. Asci 8-spored, clavate, with an apical dome which is I+ blue only in the internal, lower part, interrupted in the centre by a a paler blue strip, Candelaria-type. Ascospores 1-celled, guttulate, hyaline, oblong-ellipsoid, 11-16 x 4-6 µm. Pycnidia yellow, appearing as raised warts on the thallus surface. Conidia ellipsoid, 2.5-3.5 x c. 1.5 µm. Photobiont chlorococcoid. Spot tests: K-,C-, KC-, P-. Chemistry: probably calycin (major) and pulvinic dilactone (minor).
Note: a recently-described species, which in some parts of Europe, e.g. in Scandinavia, is even more common than C. concolor. Italian material of C. concolor needs revision: from a rapid screening of the TSB material it seems that in Italy this species is rarer than C. concolor.
Growth form: Foliose, narrow lobed
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)