Arthonia didyma  Körber

    Thallus very thin,  immersed, leaving below bark colour visible, pale greyish, pale grey-pink, pale ochre. Apothecia very small, disseminated as round, or short ellipsoid spots on the thallus,  0,1-0,5 x 0,01-0,3 mm, dark brown to brown-black, matt, not pruinose. Ascospores ovoid, slightly wool-ball shaped, 1-septate, 14-17 x 5-7 µm. Pycnidia, brown, immersed in thallus. Photobionte : Trentepohlia. Chemical spot tests negative. Rare, often undetected, usually on smooth bark of deciduous trees and shrubs, especially Corylus.  Cf. Arthonia spadicea with much bigger apothecia and Arthonia vinosa with orange spotted  thallus.*