Description: Thallus crustose, episubstratic, densely white-pruinose, continuous to usually areolate, sometimes poorly evident being masked by the densely crowded apothecia, often forming orbicular patches to 3 cm diam. Apothecia lecanorine, clustered, sessile or slightly immersed, 0.4-1.7 mm across, with a flat, yellowish to pale brown, often pruinose disc, and a more or less prominent, smooth to slightly crenate thalline margin. Thalline exciple corticate, prosoplectenchymatous, obscured by granules and larger crystals insoluble in K, but slowly soluble in N; epithecium yellowish brown, N-, with granules insoluble in K and in N, usually with an epipsamma (insoluble in K and soluble in N); hymenium colourless or yellowish, usually N+ purple-red; paraphyses slender, sparingly branched and anstomosing throughout, not markedly expanded at tips, incoherent in K; hypothecium colourless, without granules. Asci 8-spored, clavate, very thin-walled, with a K/I+ blue, tall tholus penetrated by a faintly amyloid apical cushion, the wall K/I-, surrounded by a blue outer layer, Lecanora-type. Ascospores 1-celled, hyaline, ellipsoid, (7-)10-14(-15) x (3-)5-7.5 μm. Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P- (forms with pannarin, never found in Italy, react P+ orange). Chemistry: without lichen substances, rarely with pannarin.
Note: a holarctic, widespread lichen found on a wide variety of calciferous or base-rich substrata including mortar, brick, roofing tiles, and walls, also in large urban areas and in archaeological areas. According to Roux & coll. (2014) the taxon is still heterogeneous, and deserves further study.