The Collaborative International Dictionary of English v.0.48:

Archipterygium \Ar*chip`te*ryg"i*um\
   ([aum]r*k[i^]p`t[-e]*r[i^]j"[i^]*[u^]m), n. [NL., fr. Gr.
   pref. 'archi- (E. arch-) + ptery`gion wing, fin.] (Anat.)
   The primitive form of fin, like that of Ceratodus.
   [1913 Webster]