Jurassic Clam

Even with its distinctive hinge line and markings, I have been unable to identify this large (15cm across) bivalve. It was only partly exposed in a nodule of shale when I found it. I thought that exposing the whole shell would be easy but as I removed the matrix, the closer I got to the shell the harder the rock became. It took a couple of hours each day for a month (and several blisters) to remove all the rock.