These marine mollusks existed throughout the Mesozoic Era, which is an interval of geological time from about 252 to 66 million years ago. This fossil is 4" wide and 3 1/4" long. It is cut in half and polished to show the inside sections, called "Septa". The other side is raw and shows the texture of the outside of the shell. This is a fossil, so all organic material has been replaced by rock over time, this rock appears to be agate and crystal.