The Collaborative International Dictionary of English v.0.48:

Hyporadius \Hy`po*ra"di*us\, n.; pl. Hyporadii. [Pref. hypo- +
   radius.] (Zool.)
   One of the barbs of the hypoptilum, or aftershaft of a
   feather. See Feather.
   [1913 Webster]