What is the upper part of a turtle's shell called?

The upper part of a turtle’s shell is called the carapace. The lower, belly-side portion is the plastron, and the two are joined along the sides by a bridge.

A carapace is not merely a layer of armor sitting on top of the animal. It incorporates modified ribs, vertebrae, and dermal bones, all fused into a protective structure. The turtle’s shoulder and pelvic girdles are positioned inside the shell, an unusual arrangement among vertebrates.

The outer surface is covered by scutes, which are plates made of keratin—the same tough material found in human fingernails. Scutes are often mistaken for the shell itself, but they cover the underlying bony carapace. Some aquatic turtles, including softshell turtles and leatherback sea turtles, have reduced or absent scutes and a more flexible shell surface.

The word “carapace” is also used for the upper protective covering of crustaceans and some arachnids. In turtle anatomy, however, it specifically means the dorsal, convex half of the shell; “plastron” is the correct term for the underside.