Distribution: N - Frl, VA (Matteucci & al. 2013, 2015c). C - Tosc (Tretiach & al. 2008). S - Si.
Description: Thallus crustose, episubstratic, pale to dark chestnut brown, sometimes grey-brown, areolate-subsquamulose, forming patches up to 8 cm in diam. Areoles round or angular, sometimes indistinctly lobed, 0.4-3 mm wide, up to 1 mm thick, contiguous to dispersed, flat to strongly convex, smooth or fissured and rugulose, epruinose; lower surface dark along the margins. Upper cortex paraplectenchymatous, the cells (3-)4-5(-6) µm wide, arranged in more or less vertical rows; algal layer continuous; medulla white. Apothecia lecanorine, 0.4-2 mm across, 1(-4) per squamule, when mature often larger than the corresponding areole, round to weakly angular, at first immersed, then subsessile, with an expanded, flat to slightly convex, smooth to rugose, reddish brown, shiny disc, and a thin, prominent concolorous margin. Proper exciple 15-20 µm wide; epithecium yellowish brown, 10-15 µm high; hymenium colourless, 60-90 µm high; paraphyses 2-3,5 µm thick at base, the apical cells to c. 5 µm wide; hypothecium colourless. Asci c. 200-spored, clavate, the apical dome K/I-. Ascospores 1-celled, hyaline, ellipsoid, 3-6 x 1.5-2.5(-3) µm. Photobiont chlorococcoid. Spot tests: thallus and medulla K-, C-, KC-, P-. Chemistry: without lichen substances.
Note: less calcicolous and more thermophilous than the typical subspecies. Certainly more widespread in the Alps. This taxon does not belong to Acarospora s.str. (Westberg & al. 2015), and some samples could belong to A. irregularis.