The Collaborative International Dictionary of English v.0.48:

Hypocleidium \Hy`po*clei"di*um\, n.; pl. L. Hypocleida, E.
   Hypocleidiums. [NL., fr. Gr. ? under + ? a little key.]
   (Anat.)
   A median process on the furculum, or merrythought, of many
   birds, where it is connected with the sternum.
   [1913 Webster]