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