Distribution: N - TAA (Brackel 2015, 2016). C - Tosc (Brackel 2015, 2016), Abr (Brackel 2015, 2016, Gheza & al. 2021). S - Pugl (Brackel 2015, 2016), Si (Brackel 2008c, 2016).
Description: Thallus inapparent, not lichenized, forming discoloured patches on the apothecia of Myriolecis-species. Apothecia arthonioid, black, epruinose, convex and cushion-like, irregularly rounded, 0.1-0.2 mm across. Proper exciple very poorly developed; epithecium brown, K-, I+ rust-red, K/I+ blue; hymenium colourless, I+ red, K/I+ blue; paraphysoids branched and anastomosing, the apical cells swollen; hypothecium colourless. Asci (4-)8-spored, broadly clavate, semi-fissitunicate, with a large apical dome and a distinct ocular chamber, Arthonia-type. Ascospores 1-septate, with a cell wider and longer than the other, not or only slightly constricted at septum, at first hyaline but soon turning pale brown, soleiform, 9-13 x 4-5 μm, smooth-walled. Photobiont absent. Spot tests: thallus K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: this lichenicolous fungus is widespread and quite common in Europe and is also known from Asia, North America, northern Africa, and New Zealand. It seems to be restricted to Myriolecis-species, especially M. dispersa s.lat.. It seems to be restricted to taxa of the Myriolecis dispersa-group, whereas the very similar Arthonia lecanorina grows on Lecanora albella. Collections of Arthonia clemens (which is restricted to Rhizoplaca) on epilithic Lecanora species were reported from several localities in Italy; they may belong to A. galactinaria or to A. apotheciorum, which grows on Myriolecis albescens.