gnathostegite

from The Century Dictionary.

• noun In Crustacea, a lamellar expansion of the ischiopodite and meropodite of the external maxilliped or third thoracic limb, which with its fellow covers the other mouth-parts. It may be terminated by a small jointed endognathal palp.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) One of a pair of broad plates, developed from the outer maxillipeds of crabs, and forming a cover for the other mouth organs.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology, rare One of a pair of broad plates, developed from the outer maxillipeds of crabs, and forming a cover for the other organs in the mouth.