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.

References: 
Smith, C.W. et al. editors (2009) The Lichens of Great Britain and Irelandd
Dobson, F.S. (2018) Lichens: an Illustrated Guide to the British and Irish Species