In the second place, we very frequently meet with fossils in the
state of "casts" or moulds of the original organic body. 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. In the case, in
fact, of many univalve shells, the interior cast or "mould" is
so unlike the exterior cast, or unlike the shell itself, that
it may be difficult to determine the true origin of the former.
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