Arthothelium dictyosporum

Thallus immersed, often delimited by a brown line, whitish, brown-grey or fawn; photobiont trentepohlioid. Apothecia 0.3–0.8 × 0.14–0.3 mm, irregularly rounded, polygonal or shortly elongate, flat or slightly convex, sometimes disintegrating at the centre, blackish, not pruinose, 80–190 µm tall; epithecium brown, K+ greenish; hymenium 60–90 µm tall, colourless; hypothecium 10–85 µm tall, colourless or pale brownish (K+ greenish) in parts; paraphysoids 1–1.5 (–1.8) µm diam., numerous, mostly thickened by pigment to 2.5 µm diam. in the epithecium and a few with apical caps. Ascospores 28–33 × 9–12 µm, obovoid-cylindrical, muriform, the apical cell enlarged, the lower part with 5 to 7 transverse septa and most cells with 1 or 2 longitudinal septa, when old brown and warted. Pycnidia 50–60 µm diam., few, the wall reddish brown, K+ greenish; conidia 4.5–6.5 × 0.6–0.8 µm, bacilliform.

Similar in appearance to Arthonia ilicina, but ascospores have a muriform ‘tail’ and are distinctly warted when brown, and to Arthothelium macounii which has K+ purplish and magenta pigments in the epithecium and hypothecium.