The fossils we found were solid. Kind of oval shaped, but had the shell markings on both sides, like the logo for Shell gas. Once in a while, we would only find half, but not too often. Most of the time, we had the whole thing. I was surprised at how heavy they were.
It was amazing how popular they were for show-and-tell...
Shells I found were kinda Ginko leaf shaped.
Don't know what kind of shell they are offhand, not well versed on fossil species.