Collema fuscovirens (With.) J.R. Laundon
One of the "jelly lichens" with a deep-lobed thallus, dark olive-green to blackish and paler below. Slightly translucent when wet. The minute, crowded isidia give the thallus a scabrid, matt appearance when dry. Found on well-lit, wet (often running water) limestone. Uncommon in the islands where the rocks are predominantly siliceous.
Smith et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson (2018) Lichens: an Illustrated Guide to the British and Irish Species
Ahti et al. editors (2012) Nordic Lichen Flora. Volume 3: Cyanolichens