1. The dorsal part of the exoskeleton of some crustaceans (e.g. crabs), which spreads like a shield over several segments of the head and thorax.

2. The domed dorsal part of the shell of tortoises and turtles, formed of bony plates fused with the ribs and vertebrae and covered by a horny epidermal layer. The ventral part of the shell (plastron) is similar but flatter.