Arthonia (Opegrapha) atra

One of the more frequently encountered lirellate, crustose lichens. It is characteristic on smooth bark, growing on trunks, branches and twigs, readily colonising young trees where it can form complex mosaics. It is much less frequent on nutrient-enriched bark. The lirellae are typically short and prominent with a central slit, crowded on a thin whitish or green-tinged thallus. Pycnidia are rare and spores are 3-septate.

Listed under Opegrapha atra by Alan Orange in Flora of Glamorgan (Wade et al, 1994)