Description: Thallus crustose, endosubstratic to thinly episubstratic, continuous or finely cracked, whitish to pale grey, sometimes with a pink hue,usually not well-delimited, I+ blue. Apothecia lirelliform, black, epruinose, straight or curved, simple or rarely branched, (0-4)0.8-1.5(-3) x 0.2-0.4 mm, isolated or more commonly forming star-like clusters, protruding from thallus, with a slit-like disc and a black, carbonaceous margin. Proper exciple closed below the hymenium, K+ greenish, the part below the hymenium column-like, usually taller than the upper part; epithecium brown, K+ greenish; hymenium colourless, I+ blue throughout, K/I+ blue, 60-75 µm tall; paraphyses much branched, not capitate; hypothecium black. Asci 8-spored, broadly clavate to subglobose, semi-fissitunicate, Arthonia-type. Ascospores 3-septate with rounded ends, hyaline, (11-)14-20(-21) x (3-)4-5(-6) μm, with a thin, usually poorly visible perispore. Pycnidia black, semi-immersed. Conidia straight, 5-8 x c. 1 µm. Photobiont trentepohlioid. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: on limestone, brick, roofing tiles, etc. in sheltered situations, the most common calcicolous species of the genus throughout Italy, with a wide altitudinal range. According to Roux & coll. (2014) A. trifurcata is an independent species: some records could refer to that species.