While going through my finds from the Silica Shale of Sylvania, OH I came across a number of individual valves from Strophodonta. They are preserved well enough to show the internal structure of the shells where they muscles were attached.
This is a Brachial valve with some large muscle attachment structures. At the very top of the shell is a "Y" shaped structure that fit into a corresponding notch in the Pedicle valve. The main muscles that opened and closed the shell were anchored to the central scars.
This is the interior of a Pedicle valve and you can see some palmate scars in the shell structure that correspond to the raised areas of the Brachial valve pictures above. Notes the two kidney shaped, shallow depressions near the middle of the shell. This is where the "Y" shaped structure from the Brachial valve would be when the shells were together.
This is a different shell than the one pictured above. I chose this shell since it shows more of the hinge and the teeth that were used to help connect the Pedicle and Brachial valves together.