Clams, Snails, and Cephatopods are members of a group of animals called Mollusks, which all have shells of varying size and shape. Because fossils are generally the preserved hard parts of ancient life, more is known about this group of animals than any other.
Clams are unique among mollusks in that they are adapted to filter-feeding. Also, they have two halves to their shells like brachiopods, which they can open and close on a muscular hinge, unlike brachipods, however, clams do not have a stalk. They use a muscular "foot" to move them around.
Clams have gills, like fish, which they use for breathing and straining food. Fossil clams are usually casts of the internal form of the shell. They generally do not show the fine lines and ridges that are found on the shell of a brachiopod.