The Collaborative International Dictionary of English v.0.48:

Mesoseme \Mes"o*seme\, a. [Meso- + Gr. ? sign, mark; cf. F.
   m['e]sos[`e]me.] (Anat.)
   Having a medium orbital index; having orbits neither broad
   nor narrow; between megaseme and microseme.
   [1913 Webster]