What occurs in this case will be readily understood if we imagine any common bivalve shell, as an Oyster, or Mussel, or Cockle, embedded in clay or mud.

If the clay were sufficiently soft and fluid, the first thing would be that it would gain access to the interior of the shell, and would completely fill up the space between the valves.

The pressure, also, of the surrounding matter would insure that the clay would everywhere adhere closely to the exterior of the shell.

If now we suppose the clay to be in any way hardened so as to be converted into stone, and if we were to break up the stone, we should obviously have the following state of parts.
The clay which filled the shell would form an accurate cast of the _interior_ of the shell, and the clay outside would give us an exact impression or cast of the _exterior_ of the shell (fig.
1).

We should have, then, two casts, an interior and an exterior, and the two would be very different to one another, since the inside of a shell is very unlike the outside.