A third type of fossil is a cast or a mold fossil. Internal molds are fossilized replicas of the internal structure of an organism. (Fossil on the right is an internal mold) The best examples of this type of fossil are clams or brachiopods. When a clam dies, over time the soft parts of the animal decay. The shell is left empty and this becomes filled with sediment. Eventually the shell dissolves away, but the sediments have hardened and remain as a fossil. The difference in casts and molds lies in what happens when the shell dissolves away. If the shell dissolves before its empty cavity is filled it leaves a void in the surrounding rock, which then becomes filled with sediment. The "cast" fossil that forms in the cavity shows signs of the outer shell features, while an internal mold fossil will only show signs of the shell's internal features.