In the anatomy of turtles (including tortoises and terrapins), the carapace is the larger, rounder part of a turtle shell, and together with the plastron is an integral part of a turtle's skeleton. In an ordinary turtle crawling in a prone posture, the carapace is the top part of the shell.
In a humanoid mutant turtle, the carapace comprises the turtle's back below the neck and above the legs. Both the carapace on top and the plastron on bottom are part of a turtle's shell, but because of its overwhelming visual prominence, the carapace is sometimes mistaken for being the turtle's entire shell; for example in Chuck Lorre's theme song for the 1987 TV series whose lyrics refer to the carapace as a half-shell.
The carapace of the TMNT is usually a darker brown compared to the lighter yellow tones of the plastron in the back.