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

References: 
Ahti, T. et al. editors (2012) Nordic Lichen Flora, Volume 3: Cyanolichens
Smith, C.W. et al. editors (2009) The Lichens of Great Britain and Ireland
Dobson, F.S. (2018) Lichens: an Illustrated Guide to the British and Irish Species