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I noted a comment on another ob. of a saddle oyster that they had to be opened to get a definite ID. In order to see the inside of the shell.
You may already have looked at this one.
Ive seen these many times on lobster shells, scallop shells etc, didnt know there were different types.
You may wish to take a look here http://www.ispotnature.org/node/381156.
Can you remember the size - it is significant.
here is a good description and some typical photos http://www.marlin.ac.uk/speciesinformation.php?speciesID=2527#
'Mine' are prolific here but I have never seen one larger than 11mm.
THREE MARINE PROJECTS
Lat/Lng: 51.56654, -3.97809
OS grid ref: SS629871