Description: Thallus crustose, epilithic, rather thick (0.5-2 mm), smooth to wrinkled or cracked-areolate, dull, whitish, ochre-grey or very pale brown, forming patches to 30 cm across. Apothecia lecanorine, round to (more rarely) irregular, 2-6 mm across, with a first concave, then flat, pink-ochre, finely white-pruinose disc, and a very thick, smooth, prominent thalline margin. Cortex of the thalline margin well developed, in upper part up to 70 μm, with a well differentiated medulla; proper exciple well visible in section, superficially often forming a ring around the disc; epithecium pale brown, granular, the granules dissolving in K; hymenium colourless; paraphyses thin, densely branched and anastomosed; hypothecium colourless to yellowish. Asci (4-)6-8-spored with thick, amyloid walls, Pertusaria-type. Ascospores 1-celled, hyaline, broadly ellipsoid, 45-70(-85) x (12-)25-40(-50) µm. Photobiont: chlorococcoid. Thallus: K-, C- or C+ yellow, KC+ yellow, P-; disc of apothecia K-, C+ red, KC+ carmine red, P-. Chemistry: gyrophoric acid in the epithecium, variolaric acid in thallus, often with the murolic acid complex.
Note: closely related to O. pallescens, but silicicolous and much more common, especially in Tyrrhenian Italy, where it is locally very abundant. See also note on O. pallescens.
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly sexual
Most common in areas with a humid-warm climate (e.g. most of Tyrrenian Italy)