Arthonia arthonioides (Ach.) A.L. Sm.

The extensive thallus is off-white with often a pinkish hue when fresh. It is thin and immersed in the substrate on bark or wood. Rock-growing individuals often have a soft scurfy thallus up to 1 mm thick that is occasionally dispersed into warts up to 0.8 mm in diameter. The thallus margin is not determinate. The photobiont is a species of the family Trentepohliaceae.

The apothecia are maculate, rounded or elliptical, and without pruina. They are weakly convex, 0.15–1.2 × 0.2–0.6 mm in size and 140–350 μm tall.

The epithecium is 5–7 µm tall and dark brown.

The hymenium is pale reddish brown and 40–60 μm tall.

The hypothecium is dark brown and 100–300 μm tall.

The paraphysoids are 1–1.5 μm wide. Their tips are slightly widened by dark pigment caps and plaques.

The asci are broadly clavate to subglobose, with stipe, 27–35 × 17–23 µm in size, and 8-spored.

The spores are colorless, narrowly obovoid to cylindrical, 12–16 × 4–6 μm in size, and divided by (2–)3 transverse septa. The apical cell is enlarged.

The pycnidia are brownish black, immersed in the thallus, and 100–150 µm in size. The wall is brown. The rod-shaped conidia are 5–7 × 1.5–2 μm in size and straight.

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 not been observed in the asci.

The brown pigment in the apothecia and in the wall of the pycnidia changes to green in K solution.