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Casts and molds

In some cases, the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism-shaped hole in the rock is called an external mold. If this void is later filled with sediment, the resulting cast resembles what the organism looked like. An endocast, or internal mold, is the result of sediments filling an organism's interior, such as the inside of a bivalve or snail or the hollow of a skull. Endocasts are sometimes termed Steinkerns, especially when bivalves are preserved this way.