4 and 6cm long.
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Do you have a view on JBs ID? He is usually very good at this sort of thing.
It appears to have the 'crimp' mark associated with Duck Mussel (Anodonta anatina)
However it is associated with slow rivers or lakes (as you said about Swan - Anodonta cygnea)
John - what feature or instinct triggered the ID?
Edit - Spelling
THREE MARINE PROJECTS
I think Margaritifera is more parallel-sided. It doesn't have the obtuse angle pointing to the coin. There is nothing unusual about finding Anodonta in slow rivers so long as there is some soft sediment for them to burrow into. I wouldn't want to stick my neck out so far as to say which Anodonta it is.
Lat/Lng: 53.3448, -6.527
OS grid ref: SF987926