Arthonia fusca (A. Massal.) Hepp

The thin thallus is pale fawn, dark olive-grey or olive-brown. It is minutely scurfy-granular or minutely warty. The warts are 0.15–0.3 mm in size. The thallus margin is not determinate. The photobiont is a chlorococcoid alga.

The apothecia are more or less convex and often grouped. They are irregularly rounded, sitting on the thallus, dark brown to black, and without pruina. They are 0.2–0.5 mm in size and 70–190 μm tall.

The epithecium is up to 15 µm tall and reddish brown.

The hymenium is colorless or pale green, and 35–60 μm tall.

The hypothecium is dark reddish brown and up to 120 μm tall.

The paraphysoids are 1.5–2 μm wide. Their tips are slightly widened to 2–3 µm and covered with dark pigment caps and plaques.

The asci are clavate, 32–45 × 12–18 µm in size, and 8-spored.

The spores are colorless, obovoid to slipper-shaped, often slightly constricted at the septum. They are 11–18 × 4–7 μm in size and divided by 1 transverse septum.

The pycnidia are brownish black, immersed in the thallus, and ca 40–50 µm in size. The wall is reddish brown. The rod-shaped conidia are 4.5–6 × 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+ pale blue. A KI+ blue ring structure has been observed in the asci.

The brown pigment in the apothecia and in the wall of the pycnidia changes to dull- or olive brown in K solution.