Arthonia radiata (Pers.) Ach.
Thallus crustose, rather thin, more or less immersed in the bark, 1 -3 cm in maximum length, and often mosaic-forming, smooth, white, pale grey, sometimes with a brown or olive tinge; thin brown prothallus. Apothecia more or less immersed in the thallus, flat or slightly erumpent, variably shaped, simple to long to substellate, 1-2 mm in diam., black, not pruinose. Ascospores colourless, oblong-ovoid to oblong, 3-(5) septate; apical cell not enlarged and sometimes narrowed, 12-18 x 4-6 µm. Photobiont : Trentepohlia. Chemical spot tests negative. Widespread and common mostly on smooth bark of many trees and shrubs in humid woodlands, notably Fraxinus.*