Synonyms: Allarthonia fusca (A. Massal.) Sandst.; Allarthonia lapidicola auct. non (Taylor) Zahlbr.; Allarthonia lapidicola var. ruderella (Nyl.) B. de Lesd.?; Arthonia koerberi (J. Lahm) Malbr.; Arthonia lapidicola auct. non (Taylor) Branth & Rostr.; Arthonia ruderella Nyl.; Arthonia vagans Almq. var. koerberi (J. Lahm) Almq.; Catillaria ooliticola Walt. Watson; Coniangium fuscum (A. Massal.) A. Massal.; Coniangium rupestre Körb.
Distribution: N - Ven (Nimis 1994, Lazzarin 2000b), TAA, Lomb, Piem (TSB 32906), Emil (Nimis & al. 1996, Fariselli & al. 2020), Lig. C - Tosc, Marc (Nimis & Tretiach 1999), Abr (Nimis & Tretiach 1999, Gheza & al. 2021), Mol (Nimis & Tretiach 2004, Caporale & al. 2008), Sar. S - Camp (Aprile & al. 2003b), Pugl (Nimis & Tretiach 1999), Bas (Nimis & Tretiach 1999), Cal (Puntillo 1996), Si (Caniglia & Grillo 2005, 2006).
Description: Thallus crustose, thin and mostly poorly evident, endosubstratic or thinly episubstratic and then continuous to scurfy, grey-brown. Apothecia arthonioid, immarginate, innate to adnate, angular or rounded, up to 0.5(-0.8) mm across, black, epruinose, flat to convex. Proper exciple poorly developed; epithecium reddish brown to brown-black, up to 15 μm high; hymenium colourles to greenish yellow in upper part, (35-)50-60 μm high, not inspersed with oil droplets, 1+ red, K/I+ blue; paraphysoids loose, (1-)1.5-2 μm thick, slightly branched in upper part, the apical cells up to 3.5 μm wide, with a dark cap; hypothecium reddish brown to olive-brown. Asci 8-spored, broadly clavate to subglobose, semi-fissitunicate, with a large, non-amyloid apical dome, and a distinct ocular chamber, Arthonia-type. Ascospores 1-septate, constricted at septum, with subequal cells, hyaline, elongate-ovate to slightly sole-shaped, 11-18 x 4-7 μm. Pycnidia red-brown, immersed. Conidia bacilliform, 4-6 x c. 1 μm. Photobiont absent or species associated with green algae (perhaps lichenicolous, the algae being those of the host). Spot tests: K-, C-, KC-, P-. Chemistry: without lichen substances.
Note: a holarctic species of calcareous rocks and mortar, most frequent on pebbles, but also on walls, roofing tiles etc.; in the eu-Mediterranean belt it is replaced by A. calcicola. According to Roux (in litt.) this species might not be lichenised, and grows on poorly developed crustose lichens.