The Collaborative International Dictionary of English v.0.48:

Ichthyopterygium \Ich`thy*op`te*ryg"i*um\, n. [NL., fr. Gr.
   'ichqy`s, -y`os, a fish + ? a fin.] (Anat.)
   The typical limb, or lateral fin, of fishes.
   [1913 Webster]