The Collaborative International Dictionary
Archipterygium
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.
Wiktionary
archipterygium
n. (context anatomy English) A primitive fin, like that of Ceratodus.