Inside and out
Most of the shells in the Portland Limestone are preserved as casts of the inside of the shell itself, or as an external mould of the outer shell. This block contains both; the two obvious fossils are internal casts and the one bottom-left is an external mould showing the decoration on the original shell. In all three, the actual shell has been dissolved away leaving a cast or a mould of the shell. The internal casts show the muscle scar, the pallial line (where the soft body joined the shell), and the hinge where the two shells fitted together.
Find out more about bivalves on the molluscs pages.