A lot of these on the front, lower eulittoral.
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Just found this - it should be tagged Marine. An excellent picture Graham. The key feature is evident "...telson tapers sharply towards a long pointed tooth" Peter Hayward. And it's green, though they aren't always.
I'd like this in the Project but can you remove the barnacles (please?) for now?
PS Quoted here Graham http://www.ispotnature.org/node/135961
Lat/Lng: 51.3831, 1.3262
OS grid ref: TR315701