Synonyms: Acarospora discreta sensu Bagl. & Carestia non (Ach.) Th.Fr.; Acarospora impressula var. freyi (H. Magn.) Clauzade & Cl. Roux; Acarospora smaragdula var. foveolata Bagl. & Carestia
Distribution: N - Lomb (Anzi, Lich. rar. Langob. exs. 563), Piem (Morisi & Sereno 1995, Isocrono & al. 2004), VA (Piervittori & Isocrono 1999). C - Tosc. S - Bas (Nascimbene & al. 2021), Cal (Nascimbene & al. 2021).
Description: Thallus crustose, episubstratic, dark brown to black, consisting of 0.5-1.5 mm wide, contiguous, convex areoles, the marginal ones often elongate and slightly radiating, forming up to 2 cm wide patches on the thalli of Aspicilia candida and A. polychroma. Cortex dark brown in upper part, colourless in lower part; algal layer more or less continuous; medulla white, very thick. Apothecia lecanorine, 0.2-0.5 mm across, of very different shapes on the same thallus (rounded, elongate, angular), immersed, several per areole, with an initially punctiform, then slightly expanded, smooth, concave disc concolorous with thallus, and a thick, prominent thalline margin. Proper exciple indistinct or very thin; epithecium brown; hymenium colourless, 60-80(-95) µm high, I+ dark blue; paraphyses stout, 2-2.5 µm thick at mid-level, the apical cells up to 3 µm wide; hypothecium colourless. Asci c. 100-spored, clavate, the apical dome K/I-. Ascospores 1-celled hyaline ellipsoid to broadly ellipsoid, 3-4(-5.5) x (1.5-)2-3 µm. Photobiont chlorococcoid. Spot tests: cortex and medulla K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: probably overlooked and more widespread, both in the Alps and in the Apennines, with optimum near and above treeline, this lichen starts the life-cycle on Aspicilia candida and A. polychroma on calciferous rocks which are at least partly decalcified on the surface.