Phylum: Chlorophyta   Family: Ulvaceae

Ulva rigida C. Agardh, 1823

Sea Lettuce

Widespread but erratically recorded. Growing mainly on rocks in the middle to lower shore. The toothed edge (seen with a hand lens) is a useful character

References: 
Brodie, Maggs & John (2007) - Green Seaweeds of Britain and Ireland