A shell is a hard, rigid outer layer, which has evolved in a very wide variety of different animals, including mollusks, sea urchins, crustaceans, turtles and tortoises, armadillos, etc.
Scientific names for this type of structure include exoskeleton, test, carapace, and peltidium.
The shells that are perhaps most familiar and most commonly encountered, both in the wild and for sale as decorative objects, are seashells, more precisely, the external shells of marine mollusks.
These are usually primarily composed of calcium carbonate, in the form of calcite or aragonite crystallised out in an organic matrix.
Calcium carbonate in mollusc shells can take different crystalline forms, one being nacre otherwise known as mother of pearl.
Other kinds of animal shells are made from chitin, bone and cartilage, or silica.