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

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 (2012) Nordic Lichen Flora. Volume 3: Cyanolichens