The Collaborative International Dictionary
Gnathotheca
Gnathotheca \Gnath`o*the"ca\, n.; pl. Gnathothec[AE]. [NL., fr. Gr. gna`qos the jaw + ? a box.] (Zo["o]l.) The horney covering of the lower mandible of a bird.
Wiktionary
gnathotheca
n. The horny covering of the lower part of a bird's beak.