K. Svenska Vetensk.-Akad. Handl., ser. 3, 7, 4: 282, 1929.
Synonyms: Acarospora gallica var. devastata (Eitner) H. Magn.; Acarospora hungarica H. Magn.
Distribution: N - VG (Castello 2002, Martellos & Castello 2004), TAA (Nascimbene & al. 2021), Piem (TSB 34247), Emil (Nimis & al. 1996, Fariselli & al. 2020), Lig (Giordani & al. 2016). C - Tosc.
Description: Thallus crustose, epilithic, of dispersed or contiguous areoles, pale to dark brown, epruinose, matt. Areoles 0.2-0.6(-1.5) mm wide, 0.2-0.5 mm thick, angular to round, broadly attached, sometimes reduced to a thin rim around a single apothecium, becoming subsquamulose and lobulate in well-developed specimens. Lower surface pale brown or white in well-developed areoles. Cortex 20-35 μm thick, paraplectenchymatous; algal layer continuous; medulla white, up to 100 μm thick. Apothecia 1-12 per areole, 0.1-0.6 mm across, with a usually smooth, brown (reddish when wetted), epruinose disc; thalline margin indistinct. Exciple thin, to 10–15 μm wide; epithecium reddish brown; hymenium colourless, 75-100(-130) μm tall, I+ red or blue; paraphyses 1.5-2.0 μm thick, mostly simple, the apices slightly expanded to c. 3 μm; hypothecium colourless. Asci 100-200-spored, clavate, the apical dome K/I-. Ascospores 1-celled, hyaline, narrowly to broadly ellipsoid, (2)4-5(–6.5) x 1.5.2 μm. Pycnidia usually visible as brown-red dots, globose. Conidia hyaline, simple, mostly 2.5 x 1 μm. Photobiont chlorococcoid. Spot tests: cortex and/or medulla K-, C+ red, KC+ red, P-. Chemistry; gyrophoric and lecanoric acids.
Note: a probably holarctic species of base-rich, weakly calciferous siliceous substrata, such as calcareous sandstone, brick, and roofing tiles, usually at relatively low elevations; much overlooked or confused with other species and certainly more widespread in Italy. For further details see Knudsen & Kocourková (2012).