Arthothelium ruanum (Massal.) Körb.

The thallus is thin, continuous to cracked, and pale grey, pale olive-grey or brownish grey. The margin is not determinate or bordered by a thin brown line. The photobiont is a species of the family Trentepohliaceae.

The apothecia are either level with the thallus surface or indistinctly raised. They are rounded, angular or lobed, brownish black, and do not have pruina. They are 0.4–1.6(–2) mm in size and 70–110 μm tall. The apothecia regularly disintegrate in the centre and regenerate into aggregations of individual apothecia that are characteristic for the species.

The epithecium is 10–15 μm tall, dark reddish brown, with pigment granules.

The hymenium is colorless to pale reddish brown and 35–60 μm tall.

The hypothecium is dark reddish brown and up to 30 μm tall.

The paraphysoids are 1–1.5 μm wide. Their tips are slightly widened to 2–3 µm, free, and lack distinct apical caps.

The asci are clavate, 33–60 × 15–26 µm in size, and 8-spored.

The spores are colorless at first and turn brown with granular ornamentation when old. They are narrowly obovoid, 15–26 × 7–10 μm in size, and divided by 5–8 transverse septa and 1–3 longitudinal septa. An enlarged and undivided apical cell is not developed.

Pycnidia are brownish black, are either immersed in the thallus or slightly raised, and 60–100 µm in size. The wall is reddish brown. The rod-like conidia are 4–6 × 0.8–1 μm in size.

The thallus does not react with C, K, KC, Pd, or UV (C–, K–, KC–, Pd–, UV–). Lichen secondary compounds have not been detected by TLC.

The hymenium reacts I+ red and KI+ deep blue. A KI+ blue ring structure has been observed in the asci.

The brown pigment in the epithecium and in the wall of the pycnidia changes to greenish in K solution.