Pale grey thick, warted, or areolate, light grey, sometimes with grey prothallus. Apothecia have black discs with persistent, thin, entire margins. Widespread, found on siliceous rocks, especially by lakes, seashores and nutrient-rich sites such as bird-perching stones.

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