Phylum: Ochrophyta Family: Himanthaliaceae
Himanthalia elongata (Linnaeus) S.F. Gray, 1821
Thong Weed
A common seaweed of the low-water zone. The actual frond is a button-like strcture with a short stipe. The thongs are the reproductive bodies which grow from the buttons
References:
Fish & Fish (2011) - A Student's Guide to the Seashore;
Bunker, et al - Seaweeds of Britain and Ireland