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I am fairly confident it is one of the Aplysia sea hares my students found quite a few this week rockpooling in the Falmouth area. The best way to add scale when your out is a UK coin if you don't have a ruler as everyone knows what size the coin is. There are a few species and size is one of the important ID features, also habitat can be important so if you can also get a shot of any marine species where it was found that can be very helpful
To find out about what's going on in the south west check out http://www.ispot.org.uk/forum/8409
Lat/Lng: 50.156, -5.071
OS grid ref: SW807329