It’s a half, I guess... my wife found this image... so, it’s probably not prehistoric like I think... It’s an abalone...
http://www.stylepinner.com/abalone-shell-value/YWJhbG9uZS1zaGVsbC12YWx1ZQ/
According to what I found, although they are still around today, they date back to the late cretaceous. In other words, to the time just before the dinosaurs became extinct. About 10 million years before their extinction.
The shell in your photos looks pretty old, like it could be in the millions of years, particularly with those tiny impressions of other creatures embedded in the surrounding rocky material.