The Collaborative International Dictionary of English v.0.48:

octamerous \oc*tam"er*ous\ (ocr/k*t[a^]m"[~e]r*[u^]s), a. [Octa-
   + Gr. me`ros part.] (Biol.)
   Having the parts in eights; as, an octamerous flower;
   octamerous mesenteries in polyps.
   [1913 Webster]