Plate from 18th century encyclopedia showing various shell forms. A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The term seashell usually refers to the exoskeleton of an invertebrate (an animal without a backbone). Most shells that are found on beaches are the shells of marine mollusks and made of calcium carbonate. Calcium carbonate is a chemical compound with the formula CaCO3. Types of shell included in the illustration are; cochlea, echinoderm, pecten, sea urchin, snail, solen, conch.