Description: Thallus crustose, episubstratic, rimose-areolate, forming small patches to 2(-3) cm in diam., often on other epilithic lichens. Areoles 0.5-1.5(-2.5) mm in diam., contiguous, sharply angular or elongate, dark brown to black-brown, slightly concave to slightly convex; lower surface dark. Medulla whitish grey, containing many crystals; algal layer continuous. Apothecia, lecanorine, deeply immersed in the areoles, angular to elongate, 0.2-0.3(-0.5) mm wide, 1-2(-5) per areole, with an initially punctiform, later sometimes elongate, concave to flat disc concolorous with thallus, and a thin, shiny, swollen thalline margin. Epithecium brown; hymenium colourless, 60-90 μm high, I+ blue; paraphyses 1.5-2 μm thick at base, the apical cells 2-4 μm wide. Asci c. 100-spored, clavate, the apical dome K/I-. Ascospores 1-celled, hyaline, ellipsoid, 3-5 x 1.5-2.5 μm. Photobiont chlorococcoid. Spot tests: thallus and medulla K-, C-, KC-, P-. Chemistry: without lichen substances.
Note: an arctic-alpine to boreal-montane, probably circumpolar species found on metal-rich rocks and roofing slates, more rarely on weakly calciferous siliceous rocks, usually in upland areas, with optimum above treeline. Probably overlooked and more widespread in the Alps. The record from Sicilia by Grillo & Caniglia (2004), being dubious, is not accepted here. The species does not belong to Acarospora s.str. (Westberg & al. 2015).
Growth form: Crustose
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly sexual
paras crustose lichens especially Aspicilia polychroma
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