The hard, rigid outer calcium carbonate covering of certain animals is called a shell. While many animals, particularly those that live in the sea, produce exoskeletons, usually only those of mollusks are considered to be shells.
Shells are very durable, and outlast the otherwise soft-bodied animals that produce them by a long time. Large amounts of shells may form sediment and become compressed into limestone. Shells that wash up on beaches are called seashells, and are collected by some enthusiasts.