Description: Thallus crustose, episubstratic, continuous to cracked at the periphery, areolate in central parts, very well delimited (sometimes by a black prothallus), forming orbicular patches on the thalli of Circinaria calcarea; the marginal part of the thallus is often radially cracked, giving it an almost placodioid appearance. Areoles angular, flat, smooth, white to pale grey. Apothecia lecanorine-aspicilioid, 0.2-1 mm across, irregular in outline, from round to elongate or angular, immersed in the areoles, with a black, often grey-pruinose, concave to flat, smooth disc, and a very slightly raised thalline margin. Epithecium greenish brown; hymenium colourless; paraphyses more or less branched and anastomosing; hypothecium colourless, subtended by a continuous algal layer. Asci (6-)8-spored, clavate, the thin outer coat K/I+ blue, the wall and apical dome K/I-. Ascospores 1-celled, hyaline, broadly ellipsoid, 8-14 x 5-10 μm, 1-2 seriate in the asci. Pycnidia immersed, black, the wall colourless in sections except around the ostiole. Conidia bacilliform, 4-5 x c. 1 μm. Photobiont chlorococcoid. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a southern European and Mediterranean-montane, very characteristic but often overlooked species described from Egypt; it is found on calcareous rocks, starting the life-cycle on Circinaria calcarea and related species. It is not rare in Italy, especially in the Apennines and on the main Islands, and should be looked for further in northern Italy. The species is chemically variable, which led to the description of several varieties whose taxonomic value should be re-assesed on the basis of molecular data, so that in the following I refrain from a formal recombination.