Source : Webster's Revised Unabridged Dictionary (1913)

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.