The Collaborative International Dictionary of English v.0.48:

Hypoarion \Hy`po*a"ri*on\, n.; pl. Hypoaria. [NL., fr. Gr.
   "ypo` beneath + ? a little egg.] (Anat.)
   An oval lobe beneath each of the optic lobes in many fishes;
   one of the inferior lobes. --Owen.
   [1913 Webster]