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.
[Illustration: Fig.

1 .-- _Trigonia longa_, showing casts to of the exterior and interior of the shell .-- Cretaceous (Neocomian).] It only remains to add that there is sometimes a further complication.

If the rock be very porous and permeable by water, it may happen that the original shell is entirely dissolved away, leaving the interior cast loose, like the kernel of a nut, within the case formed by the exterior cast.

Or it may happen that subsequent to the attainment of this state of things, the space thus left vacant between the interior and exterior cast--the space, that is, formerly occupied by the shell itself--may be filled up by some foreign mineral deposited there by the infiltration of water.

In this last case the splitting open of the rock would reveal an interior cast, an exterior cast, and finally a body which would have the exact form of the original shell, but which would be really a much later formation, and which would not exhibit under the microscope the minute structure of shell.
[Illustration: Fig.