A thin, encrusting lichen, mid to dark green, found on rocks of the lower intertidal zone. Often with a white prothallus, it has a smooth, oily appearance. Apothecia are black, largely embedded in the thallus but more obvious when dry

Determiner: 
Rebecca Yahr

References: 
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species