Entry 1 sense Webster, 1913 Archipterygium /(är*kĭp`tē̇*rĭj"ĭ*ŭm)/ · Ar·chip·te·ryg·i·um · IPA 01 n. The primitive form of fin, like that of Ceratodus. 1. The primitive form of fin, like that of Ceratodus.(Anat.)