The Collaborative International Dictionary of English v.0.48:

Mesopterygium \Me*sop`te*ryg"i*um\, n. [NL., fr. Gr. me`sos
   middle + ? a fin.] (Anat.)
   The middle one of the three principal basal cartilages in the
   fins of fishes. -- Me*sop`ter*yg"i*al, a.
   [1913 Webster]