This is Pinna nobilis, the noble pen shell.
Image on its own - http://www.idscaro.net/sci/01_coll/pics/bival/pinnidae/pinna_nobilis2a.jpg
That is from http://www.idscaro.net/sci/01_coll/plates/bival/pl_pinnidae_1.htm (search the page for 'nobilis' to see the image with annotation).
Many related species come from various corners of the globe, but this one is Mediterranean.
In fact it is 'the largest shellfish living only in the Mediterranean'.
It has been under EU protection since 1992.
It is the source of 'sea silk'