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

Determiner: 
Chris Johnson

References: 
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species
Ahti et al. editors (2002) Nordic Lichen Flora, Volume 2: Phyciaceae