Wiktionary
undershell
n. The plastron of a turtle or tortoise.
Usage examples of "undershell".
Without ceremony, she thrust the saber into the leathery undershell and disemboweled the creature.
Auris swung herself upon the edge of the undershell beside the neck, crept on hands and knees into the hollow between the upper and lower shells, Ilf climbed in after her.