ABSTRACT: As in known that the Maastrichtian stage is divided in to two chronozones by benthic foraminifers, but it is divided in to three by planktk foraminifers. On the other hand, until now Omphalocychn macroporas-Orbitoides médius biozone has always becn^ aged as Upper Maastrichtian. However,,, where it could be controlled, by planktic foraminifers » it is observed that this zone is equivalent of Middle-Upper Maastrichtian,. This situation is explained in. this paper with its proofs,.