The Collaborative International Dictionary of English v.0.48:

Megaseme \Meg"a*seme\, a. [Mega- + Gr. ? sing, mark: cf. F.
   m['e]gas[`e]me.] (Anat.)
   Having the orbital index relatively large; having the orbits
   narrow transversely; -- opposed to microseme.
   [1913 Webster] Megass