Collemopsidium foveolatum (A.L. Sm.) F. Mohr
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.
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species