Mosaic-forming areolate thallus. The black prothallus is visible between the pale to dark grey areoles. Immersed apothecia with small black discs are usually abundant. Widespread on siliceous rocks and drystone walls (dykes in Scotland). Often a pioneer species

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
Ahti et al. editors (2002) Nordic Lichen Flora, Volume 2: Phyciaceae