carapace

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Zoology A hard bony or chitinous outer covering, such as the fused dorsal plates of a turtle or the portion of the exoskeleton covering the head and thorax of a crustacean.
2. n. A protective, shell-like covering likened to that of a turtle or crustacean: "He used to worry that Sarah would age the same way, develop the same brittle carapace” ( Anne Tyler).

Century Dictionary and Cyclopedia

1. n. The shell of a turtle or tortoise; specifically, the upper shell, the under shell being called the plastron. See also cut under Chelonia.
2. n. In Mammalia, the shell of an armadillo.
3. n. In Cirripedia, the multivalvular shell, test, or case.
4. n. In higher Crustacea, the shield covering the cephalothorax, sometimes separable into a cephalostegite and an omostegite. See cut under Apus.
5. n. One of the many hard cases, tests, or shells which are likened to a carapace, as those of certain infusorians; a lorica.

Wiktionary

1. n. A hard protective covering of bone or chitin.
2. n. in figurative use

GNU Webster's 1913

1. n. (Zoöl.) The thick shell or shield which covers the back of the tortoise, or turtle, the crab, and other crustaceous animals.

WordNet 3.0

1. n. hard outer covering or case of certain organisms such as arthropods and turtles