Seashells are exoskeletons of marine invertebrates such as mollusks and sea snails, and they grow from the inside out or by depositing new material at the edges, according to Scientific American. Two common types of seashells are bivalves and gastropod shells. The former are made by mollusks such as oysters and clams. The latter are exoskeletons of marine snails.
Bivalves and gastropod shells are commonly carried ashore by waves and crushed up to form sandy beaches. Unlike turtle shells, seashells are not made of cells; instead, they are composed of minerals, such as calcium carbonate, and a small amount of proteins. Empty mollusk seashells are often used by other marine animals, such as hermit crabs, for shelter. These creatures seek newer and larger shells throughout their lifetimes as they outgrow the old ones.
In contrast, turtle shells are made of dried epidermal tissues containing keratin, the same substance that makes up human nails. The innermost part of turtle shells is composed of living tissue, including blood vessels and nerves, and minerals, the most common of which is calcium phosphate. This mineral layer is called the carapace and is created by the fusion of the rib cage and vertebrae during the development of the animal.