Description: Thallus crustose, episubstratic, rimose-areolate, forming small, up to 2(-3) cm wide patches, often on other epilithic lichens. Areoles 0.5-1.5(-2.5) mm wide, contiguous, sharply angular or elongate, dark brown to black-brown, slightly concave to slightly convex, the marginal ones not radiating; lower surface dark. Medulla whitish grey, containing many crystals; algal layer continuous. Apothecia lecanorine, deeply immersed in the areoles, angular to oval-elongate, 0.2-0.3(-0.5) mm wide, 1-3(-5) per areole, with an initially punctiform, later sometimes elongate, concave to flat disc which is usually darker than thallus, a thin, shiny, swollen thalline margin, and often a distinctict parathecial ring. Proper exciple expanding around the disc, up to 40 μm thick; epithecium brown; hymenium colourless, 60-90(-130) μm high, euamyloid, K/I+ blue; paraphyses stout, 2-3 μm thick at mid-level, the apical cells 2-4 μm wide; hypothecium colourless. Asci c. 100-spored, clavate, the apical dome K/I-. Ascospores 1-celled, hyaline, broadly ellipsoid, 3-5 x (1.5-)2-2.5 μm. Photobiont chlorococcoid. Spot tests: cortex and medulla K-, C-, KC-, P-, UV-. 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