The Collaborative International Dictionary of English v.0.48:

Urosteon \U*ros"te*on\, n.; pl. L. Urostea, E. Urosteons.
   [NL., fr. Gr. ? the tail + ? a bone.] (Anat.)
   A median ossification back of the lophosteon in the sternum
   of some birds.
   [1913 Webster]