The Collaborative International Dictionary of English v.0.48:

Hypoplastron \Hy`po*plas"tron\, n.; pl. Hypoplastra. [Pref.
   hypo- + plastron.] (Anat.)
   The third lateral plate in the plastron of turtles; -- called
   also hyposternum.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hyposternum \Hy`po*ster"num\, n.; pl. L. Hyposterna, E.
   Hyposternums. [Pref. hypo- + sternum.] (Anat.)
   See Hypoplastron. Hypostome