A plastron is the flat, hard underside of a turtle's or tortoise's shell. While the rounded top part (called the carapace) protects the animal from above, the plastron shields its belly. Together, these two pieces form the complete shell that makes turtles so distinctive.

The plastron is made of bone covered with plates of keratin, the same material as your fingernails. In many turtles, the plastron is pale yellow or cream-colored, often different from the darker carapace on top. Box turtles have a special plastron with a hinge that lets them pull it up tight against the top shell, sealing themselves completely inside when threatened.

Scientists can learn a lot from examining a turtle's plastron. Like tree rings, the plates show growth patterns that can hint at the animal's age and health history. Some species have distinctive markings on their plastrons that help researchers identify individual turtles, much like fingerprints can identify people.