The first step in identifying shell fossils is to make sure the item you are examining is a true fossil, not a modern shell. Shell fossils are formed when an animal dies and sinks to the bottom of a lake or ocean. If the animal becomes covered with dirt and is not eaten, it will be protected from erosion and other environmental hazards. As sediment accumulates, the weight compacts the underlying area. In a process called lithification, the pressure from this weight turns the dirt into sedimentary rock, and the hard parts of the animal are mineralised.
Look carefully at the shell, and consider its weight. True fossils are rocks and will be much harder and heavier than shells.
Analyse the overall shape. Fossil shells may be circular, egg-shaped, cup-shaped, tubular or spiral. Spiral shells are often snail fossils or ammonites.
Look at the orientation of the shell. Some brachiopod shells are biconvex, which means both sides of the shell curve outward. Examine the hinge of the shell, if it has one. Some hinges are flat; others are pointed.
Examine the surface of the shell. Patterns on the shell are called decoration, and they can be useful in identifying species.
Use a simple yes-no system to help identify shell fossils with the aid of a fossil field guide. Through the process of elimination, you can narrow the list of species that a particular fossil represents.
Work from general appearance to details. Museum websites often have good photographs of fossils.
Dendrites are crystal formations that resemble fossils but are not fossils. Don't collect fossils on private property unless you have permission from the owner.