Page "Fossil" Paragraph 15
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Wikipedia
In some cases the original remains of the organism have been completely dissolved or otherwise destroyed.
When all that is left is an organism-shaped hole in the rock, it is called an external mold.
If this hole is later filled with other minerals, it is a cast.
An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull.
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