Description: Thallus crustose, episubstratic, chalky white to rarely white-grey, 0.25-0.8 mm thick, finely rimose-areolate, sometimes delimited by a white to grey, more or less zoned prothallus, forming orbicular patches up to 30 cm in diam. Areoles angular, smooth, the peripheral ones often longer than wide and radially arranged. Cortex entirely filled by crystals visible under polarized light, its structure therefore poorly visible, lacking pigments; algal layer 30-65 µm high; medulla white, filled by crystals of calcium carbonate, 0.17-0.3 mm thick, I-. Apothecia lecanorine-aspicilioid, immersed, 1(-4) per areole, irregular in size and shape, from punctiform to angular or even linear, 0.15-0.7(-1) mm wide, with a concave to flat, black, sometimes slightly white-pruinose disc, and an up to 0.15 mm thick, chalky white, slightly raised, finally often excluded thalline margin. Epithecium olive-green, N+ brigher green, K+ brown; hymenium colourless, I+ blue soon turning red-brown; paraphyses septate and anastomosing, sometimes moniliform in upper part; hypothecium colourless. Asci (2-)4-spored, clavate, the thin outer coat K/I+ blue, the wall and apical dome K/I-. Ascospores 1-celled, hyaline, broadly ellipsoid to subglobose, 18-30 x 14-28 µm. Pycnidia immersed, appearing as black dots. Conidia (6-)7.5-10(-11.5) x 1(-1.5)µm. Photobiont chlorococcoid. Spot tests: cortex and medulla K-, C-, KC-, P-. Chemistry: aspicilin.
Note: a mainly Mediterranean to mild-temperate species found on limestone and dolomite, sometimes also on other calciferous substrata, much rarer in heavily disturbed habitats (stunted specimens, however, may grow on ancient monuments in the centre of Rome), sometimes reaching beyond treeline, especially in the Apennines.