Opegrapha herbarum Mont.
A thin, grey to olive or brown thallus with lirellate apothecia up to 2mm in length. Part of a difficult genus commonly requiring microscopic examination of the spores for accurate identification. A widespread and common species across the UK, but first recorded in the Outer Hebrides in 2018. Prefers sheltered sites on smooth-barked hosts
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species