These next pictures are of the Pelecypod Goniophora. The first picture shows a smaller specimen with the tell tale shell markings of regularly spaced growth lines that form a right angle across a longitudinal axis line.
These next two pictures show a larger individual that is just an internal shell cast. You can still see the signature shell shape reflected in the cast.
Casts of the internal part of the shells are neat to look at because you can see how the hinge and shell edges were shapes and fit together. This picture almost looks like one of those Rorschach images.