Anamorph: not known.
Teleomorph: ascomata 0.1–1 mm diam., often in small groups, blackish, flat or slightly convex, ± round or rarely irregular in shape, immersed in the apothecial disc or sometimes the thallus of the host lichen, often filling the entire apothecium. Exciple poorly developed. Hymenium 40-50 μm tall, colourless or pale brown, I+ red, K/I+ blue. Epihymenium brownish to olivaceous brown, intracellular pigment not granulose, K– or K+ more intense olivaceous green, N–, covered by a colourless gelatinous sheet 2-8 μm thick. Subhymenium and hypothecium well-developed, to more than 100 μm tall, colourless, I+ blue, K/I+ blue. Interascal tissue of branched and anastomosed paraphyses with short and wide cells, erect, septate, capitate (to 3–5 μm thick) at the apex, brownish to olivaceous green in the upper 3–9(–15) μm. Asci 22–42 × 10–15 μm, broadly clavate to elongate ellipsoidal, the wall strongly thickened in the upper part with a long ocular chamber, hemiamyloid ring present, but often poorly visible, (5–) 8-spored. Ascospores 10–14.5(–15) × (3.5–)4–5.5(–6.5) μm, ellipsoidal, colourless, thin-walled, (1–)2- or 3-septate, not or slightly constricted at the septa, I–, KI–, uniseptate ascospores with a median to submedian septum, perispore absent or very thin.
Not formally evaluated. Currently only known from a single GB&I collection, but the host is very common and widespread and the parasite is quite probably overlooked.
Similar to Arthonia varians, which occurs on Lecanora rupicola and has longer ascospores (13–18 × 4–7 µm versus 10–14.5(–15) × (3.5–)4–5.5(–6.5) µm). See also Arthonia lecanoricola, also recently reported from GB&I on Lecanora populicola.
In apothecia (and sometimes on the thallus) of Lecanora muralis.
In GB&I, known only from VC82 East Lothian.
Parasitic, frequently causing sterility of the host apothecia.