f 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.

[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. 2.--Microscopic section of the silicified
wood of a Conifer (_Sequoia_) cut in the long direction of the
fibres. Post-tertiary? Colorado. (Original.)]

[Illustration: Footnote: Fig. 3.--Microscopic section of the wood
of the common Larch (_Abies larix_), cut in the long direction of
the fibres. In both the fresh and the fossil wood (fig. 2) are
seen the discs characteristic of coniferous wood. (Original.)]

In the third class of cases we have fossils which present with
the greatest accuracy the external form, and even sometimes the
internal minute structure, of the original organic body, but
which, nevertheless, are not themselves truly organic, but have
been formed by a "replacement" of the particles of the primitive
organism by some mineral substance. The most elegant example
of this is afforded by fossil wood which has been "silicified"
or converted into flint (_silex_). In such cases we have fossil
wood which pre