Phylum: Ochrophyta   Family: Scytosiphonaceae

Colpomenia peregrina Sauvageau, 1927

Oyster Thief

The thallus is a hollow 'bag', variously shaped. Dry and papery to the touch and easily torn. An introduced species from the north-east Pacific, but not thought to be harmful to the local environment

References: 
Fletcher (1987) - Seaweeds of the British Isles, Volume 3 Fucophyceae (Phaeophyceae), Part 1