A mollusk's exoskeleton is composed of chitin (a component of the exoskeleton of most animals) and conchiolin which is a form of calcium carbonate (present in snail shells). These complex proteins are secreted by the mantle layer present right below the shell and hardens over time as the creature grows.
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The "nautilus" is a family of hard-shelled cephalopods, notably Nautilus pompilius, or "chambered nautilus".