Thin, pale, often greenish thallus with black perithecia. More frequent on base-rich substrate but occasionally found on siliceous rock where the thallus is generally thinner

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