Description: Thallus crustose, episubstratic, areolate-subsquamulose, often forming extensive patches. Areoles contiguous or more or less overlapping, 0.5-4 mm wide, rather thick, rounded or angular to slightly lobed, flat to convex, epruinose, smooth to slightly roughened, pale to medium brown or olive-brown, with a paler but epruinose margin, matt, the underside whitish. Algal layer 90-200 μm thick, irregular, discontinuous, penetrating the medulla in tooth-like columns; medulla greyish, filled by masses of crystals. Apothecia 0.5-3 mm across, immersed, 1(-2) per areole, with a reddish brown, pruinose, smooth disc and a thin, prominent, sometimes pruinose thalline margin. Epithecium brown, with crystals; hymenium colourless, 60-100 μm tall, I+ blue; paraphyses 2-3 μm thick at base, to 7 μm at apex, with a brown cap; hypothecium colourless, 30-50 μm tall. Asci 100-200-spored, clavate, the apical dome K/I-. Ascospores 1-celled, hyaline, cylindrical, 4-6(-8) x (1.5-)2-3(-4) μm. Photobiont chlorococcoid. Spot tests: thallus and medulla K-, C-, KC-, P-, UV-. Chemistry: no lichen substances.
Note: a widespread, probably holarctic species found on the top of more or less calcareous boulders in natural habitats, sometimes overgrowing other crustose lichens, with a wide altitudinal range but most common in upland areas; closely related to A. cervina, perhaps more frequent in less exposed situations. See also note on A. cervina.
Magnusson A. H. (1935) Acarosporaceae, Thelocarpaceae. - In: Rabenhorst G. L.: Kryptogamen-Flora von Deutschland, Österreich, und der Schweiz-. 2nd- IX. Die Flechten. Abt. 5- 1. Gebr. Borntraeger. Leipzig, pp. 1-318. – Public Domain