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The Collaborative International Dictionary
Mesopterygium

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.

Wiktionary
mesopterygium

n. (context anatomy English) The middle one of the three principal basal cartilages in the fins of fishes.