Orbitoid Foraminifera, on account of their short stratigraphic range, have proved to be excellent horizon markers, and, because of their wide geographic distribution, they are valuable in correlation. The genus Orbitoides, as now restricted, is found exclusively in deposits of Cretaceous age, Orthophragmina appears to be confined to the Eocene; but Lepidocyclina ranges through the upper Eocene and Oligocene.
Additional Publication Details
USGS Numbered Series
The American species of Orthophragmina and Lepidocyclina