elytrum

from The Century Dictionary.

• noun In entomology, the modified fore wing of beetles or Coleoptera, forming with its fellow of the opposite side a hard, horny, or leathery case or sheath, more or less completely covering and protecting the posterior membranous wings when these are folded at rest, and usually forming an extensive portion of the upper surface of a beetle; a shard.
• noun In some chætopodous annelids, as the Aphroditidæ, or polychætous annelids, as the Polynoë, one of the squamous lamellæ overlying one another on the dorsal surface of the worm, made by a modification of the dorsal cirri of the parapodia, of which they are thus specialized appendages.
• noun See the adjectives.