Widespread and commonly found on the shells of barnacles, limpets and mussels. Immersed thallus with very small, black perithecia (fruiting bodies) that are immersed in pits.

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