hypotarsus

from The Collaborative International Dictionary of English v.0.48

Hypotarsus \Hy`po*tar"sus\, n.; pl. {Hypotarsi}. [NL. See
   {Hypo-}, and {Tarsus}.] (Anat.)
   A process on the posterior side of the tarsometatarsus of
   many birds; the calcaneal process. -- {Hy`po*tar"sal}, a.
   Hypotenuse