Bacidia arceutina (Ach.) Arnold
Smooth, often immersed thallus with pale brown apothecia which become black and globulose as they mature. Found on the nutrient-rich and basic-barked shrubs and trees, occasionally on rocks. Widespread but not particularly common.
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species