The Collaborative International Dictionary of English v.0.48:

Ephippium \E*phip"pi*um\, n. [L., saddle cloth, fr. Gr. ?; 'epi`
   on + "i`ppos horse.]
   1. (Anat.) A depression in the sphenoid bone; the pituitary
      fossa.
      [1913 Webster]

   2. (Zo["o]l.) A saddle-shaped cavity to contain the winter
      eggs, situated on the back of Cladocera.
      [1913 Webster]