Arthonia stellaris Kremp.

The thin thallus is immersed in the host bark and greenish white, light grey or pale fawn. The margin is bordered by a brown line or it is not determinate. The photobiont is a species of the family Trentepohliaceae.

The apothecia are level with the thallus surface or indistinctly raised. Their shape is narrowly elongated to irregularly branched or star-shaped. The apothecia are reddish brown (when moist) to brown-black and do not have pruina. They are sometimes covered by a thin layer of bark cells. They are up to 2 mm large, with individual branches 0.08–0.15 mm wide, and 60–95 μm tall. The disc is flat.

The epithecium is 5–10 μm tall and reddish brown.

The hymenium is colorless to pale reddish brown and 40–50 μm tall.

The hypothecium is indistinct or up to 20 µm tall, colorless or pale reddish brown.

The paraphysoids are 1–1.5 μm wide. Their tips are slightly widened to 2 µm, brown pigmented or with distinct dark pigment plaques.

The asci are clavate or obovoid, 40–45 × 17–20 µm in size, and 8-spored.

The spores are colorless first, with a brown pigmentation and granular ornamentation when old. They are obovoid to narrowly obovoid, 13–24 × 5–9 μm in size, and are divided by (2–)3–4 transverse septa. The apical cell is enlarged.

The pycnidia are brownish black, immersed in the thallus, and 40–60 µm in size. The wall is reddish brown in color. The rod-shaped conidia are 4.5–5.5 × 0.5–1 μm in size.

The thallus does not react with C, K, KC, Pd or UV (C–, K–, KC–, Pd–, UV–). An unidentified lichen secondary compound called the “stellaris unkown” has been detected by TLC. 

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

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