Arthothelium macounii (G. Merr.) W.J.Noble

The thallus is whitish, pale olive grey or pale fawn. It often has a reddish hue around the apothecia. The thallus is immersed in the bark and has a smooth, weakly glossy surface. 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 or slightly raised. They are irregularly rounded or elliptical, brownish black with often a faint purplish hue, and do not have pruina. They are 0.4–1.5 × 0.4–0.8 mm in size and 80–120 μm tall. The disc is flat or slightly convex and often covered by a thin layer of bark along the margins.

The epithecium is dark orange brown or purplish brown and 15–20 μm tall. It often includes layers of bark cells.

The hymenium is colorless or pale purple and 70–90 μm tall.

The hypothecium is orange-brown or purplish brown and up to 25 µm tall.

The paraphysoids are 1–1.5 μm wide. Their tips extend horizontally along the surface of the hymenium, with dark brown pigment deposited on the outer walls. Sparse orange-red oily inclusions occur throughout the apothecia.

The asci are broadly obovoid to subglobose, with stipe, 50–60 × 35–45 μm large, and contain 8 spores.

The spores are colorless first, with a brown pigmentation and granular ornamentation when old. They are oblong with an enlarged and undivided apical cell, 20–38 × 10–14 μm large, and divided by 4–6 transverse septa and 1–2 longitudinal septa in the basal cells.

The pycnidia are brownish black, immersed or slightly raised over the thallus, and ca 40–70 µm in size. The wall is dark brown. The rod-shaped conidia are 4–5.5 × 1–1.5 μm in size and straight.

The thallus does not react with C, K, KC, Pd, or UV (C–, K–, KC–, Pd–, UV–). TLC data are not available for this species. 

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

The orange-red pigment in the apothecia dissolves in K with a clear purple solution.