Arthonia spadicea  Leighton

     Thallus crustose, very thin, more or less immersed, usually not well delimited, pale grey, pale grey-green, almost transparent and leaving bark colour visible below. Apothecia  more or less round-shaped, 1-1,5 mm in diam., flat to slightly convex, red-brown, chocolate,  to brown-black, often slightly glossy, resembling spots of tar. Ascospores ovoid or slipper-shaped, 1-septate, colourless, 7-11 x 3-4 µm. Pycnidia frequent, 0.1 mm diam., brown. Photobiont : Trentepohlia. Chemical spot tests negative except apothecia and pycnidia slightly K+ purple (weak reaction). Uncommon, on very shaded bark of trunks (usually at or near base) of basic barked, deciduous trees of moist woodlands. Cf. Arthonia vinosa with thallus orange spotted, apothecia more convex and orange and smaller ascospores; and  Arthonia didyma with much smaller apothecia.*