Description: Thallus crustose, episubstratc, areolate-verrucose, white, grey or pale brown, dull, forming irregular, poorly delimited patches. Areoles more or less dispersed or gathered in small groups, usually round, sometimes angular or irregular, first flat, then convex, (0.2-)0.5-1.2(-2.2) mm in diam. Apothecia lecanorine-aspicilioid, 0.2-0.5(-0.8) mm across, 1(-3) per areole, round, rarely angular, with a black, but usually white-pruinose, concave to flat disc and a slightly raised thalline margin. Epithecium olive green to olive-brown, with crystals, N+ green, K+ brown; hymenium colourless, I+ blue or sometimes partly turning yellow-green; paraphyses slightly branched and anastomosing, (1-)1.5-2(-2.5) µm thick in lower part, in upper part submoniliform, the uppermost cells subglobose, (3-)4-6 µm wide; subhymenium and hypothecium colourless. Asci (2-)4(-6)-spored, clavate, the thin outer coat K/I+ blue, the wall and apical dome K/I-. Ascospores 1-celled, hyaline, globose to subglobose, (16-)18-28(-35) x (14-)17-24(-28) µm. Pycnidia rare, immersed, with a black, punctiform ostiole. Conidia thread-like, straight or slightly curved, (5-)6-10(-11) x 0.8-1(-1.3) µm. Photobiont chlorococcoid. Spot tests: cortex and medulla K-, C-, KC-, P-. Chemistry: usually with aspicilin, rarely lacking secondary metabolites.
Note: a widespread lichen growing on calcreous rocks, often near the ground, which in Italy is much less frequent than C. hoffmanniana, being generally bound to less disturbed situations.