Distribution: N - VG, Ven (TSB 7983), Lig (GDOR 371). C - Tosc (TSB 21202), Laz (TSB 8395), Sar (TSB 11841). S - Cal (TSB 10553), Si (TSB 17239).
Description: Thallus endosubstratic or thinly episubstratic, continuous to cracked, usually not well-delimited, whitish to greenish white. Apothecia lirelliform, black, epruinose, straight or curved, simple or branched, 0.2-1.5 x 0.1-0.3 mm, isolated or aggregated into irregular clusters, protruding from thallus, with a slit-like disc and a black margin. Proper exciple black, carbonized, closed below the hymenium, K-, the part below the hymenium thin, at most as tall as the hymenium and subhymenium together; epithecium brownish, K-; hymenium colourless, 50-100 µm high, I+ reddish throughout, K/I+ blue; paraphysoids much branched and anastomosing, not capitate; hypothecium black. Asci 8-spored, broadly clavate to subglobose, semifissitunicate, Arthonia-type. Ascospores 3-septate with rounded ends, hyaline, (10-)13-20(-24) x 4-6(-7) μm, usually slightly restricted in the center into two unequally wide parts, the perispore very thin or absent. Pycnidia black, more or less immersed. Conidia straight, 4-10 x 0.5-1.5 µm. Photobiont trentepohlioid. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: this species has been often confused with A. calcarea, so that its distribution in Italy is still poorly known. It grows on calcareous rocks in rather sheltered situations, below the montane belt.