Phylum: Rhodophyta   Family: Ahnfeltiaceae

Ahnfeltia plicata (Hudson) E.M. Fries, 1836

Black Scour Weed

Grows on rocks in the sublittoral zone and in rockpools on the lower shore, often in association with sand where it can be partly buried. Widespread in suitable habitat

References: 
Bunker, Brodie, Maggs & Bunker (2017) - Seaweeds of Britain and Ireland