The Collaborative International Dictionary of English v.0.48:

Gnathidium \Gna*thid"i*um\, n.; pl. Gnathidia. [NL., fr. Gr.
   gna`qos the jaw.] (Zool.)
   The ramus of the lower jaw of a bird as far as it is naked;
   -- commonly used in the plural.
   [1913 Webster]