Variable, rosette forming, dark greenish thallus. Very swollen when wet. Numerous red-brown apothecia often obscure the lobes of the thallus. Common on mortar in walls

Determiner: 
Christine 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 (2012) Nordic Lichen Flora. Volume 3: Cyanolichens