Cracked-areolate, greyish to mousey-brown thallus with thin black prothallus. Black apothecia with thin, persistent margin, initially inset later becoming slightly raised. Very common throughout Britain, on pebbles and other siliceous rocks

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