Description: Thallus crustose, episubstratic, 1-1.5 mm thick, ecorticate, forming chalky white, smooth or undulate, well-delimited, up to 2-(-3) cm wide, often confluent patches, without a distinct prothallus. Apothecia arthonioid, black but often grey-pruinose, rounded to elongate, 0.7-1.5 mm across, at first immersed, then broadly sessile, flat to finally convex, without a distinct margin. Proper exciple poorly developed; epithecium green-black, 20-25 μm high; hymenium colourless to very pale brown, 50-85 μm high, I+ blue; paraphysoids thread-like, branched and anastomosing in upper part, with thicker apical cells; hypothecium well-developed, brown-black, wedge-shaped and paler brown in lower part. Asci 8-spored, broadly clavate, semi-fissitunicate, with a large apical dome, a distinct ocular chamber, and a K/I+ bluish ring-structure, Arthonia-type. Ascospores 3-septate, at first hyaline, but turning brown when overmature, with 4 unequal loculi, spindle-shaped, straight, with rounded ends, 10-13 x 5-6 μm. Pycnidia semi-immersed, black, somehow shiny. Conidia bacilliform, straight, 6-8 x c. 1 μm. Photobiont trentepohlioid. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a mainly Mediterranean species found in shaded underhangs of calcareous rocks subject to humid maritime winds.