Rhizocarpon richardii (Lamy ex Nyl.) Zahlbr.
Brown to greyish-brown, areolate-cracked thallus, often with a fimbriate blackish-brown prothallus. Inset flattish, black apothecia with thin margins. Very common on siliceous coastal rocks forming mosaics.
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species