The Collaborative International Dictionary of English v.0.48:

Pygostyle \Py"go*style\, n. [Gr. pygh` the rump + ? a pillar.]
   (Anat.)
   The plate of bone which forms the posterior end of the
   vertebral column in most birds; the plowshare bone; the
   vomer. It is formed by the union of a number of the last
   caudal vertebr[ae], and supports the uropigium.
   [1913 Webster]