The Collaborative International Dictionary of English v.0.48:

Xiphiplastron \Xiph"i*plas"tron\, n.; pl. Xiphiplastra. [NL.,
   fr. Gr. xi`fos a sword + plastron.] (Anat.)
   The posterior, or fourth, lateral plate in the plastron of
   turtles; -- called also xiphisternum.
   [1913 Webster]