A turtle’s individual shell plates made of keratin are called scutes.
Scutes are the hard, horny outer covering over a turtle’s underlying bony shell. The shell has two main regions: the carapace on the back and the plastron underneath. Scutes on the carapace include central or vertebral scutes, costal scutes, and marginal scutes, while the plastron carries named scutes such as gular, humeral, pectoral, abdominal, femoral, and anal.
Scutes are not the same as the shell’s bones. Beneath them lie bony plates formed from modified vertebrae, ribs, and other skeletal elements. Keratin is the same tough structural protein found in human fingernails, hair, and the outer layer of skin. As a turtle grows, new keratin layers are added at the bases of many scutes.
Aquatic turtles commonly shed individual scutes, while many terrestrial tortoises retain them and build visible growth layers. “Plastron” is therefore a tempting but incorrect alternative: it names the lower half of the shell, not an individual plate. “Osteoderms” are also different; they are bony skin deposits found in various reptiles and are not the standard name for turtle shell plates.