Planktonic Foraminifera occur abundantly in most California "Miocene" strata, but have not been studied previously in detail. By using these planktonic forms, 11 zones are recognized and are related to the existing "Miocene" stages which are based on benthonic species. Correlation of these zones with the standard reference sections of tropical regions is complicated by paleoecologic factors, which probably include regional variation in water temperature between the tropics and California. Stratigraphically lower zones correspond more closely than do higher ones, indicating a general cooling of the California seas throughout middle Cenozoic time.

Species of Globigerina and Eoglobigerina are abundant in the California "Miocene," whereas species of Turborotalia, Globoquadrina, Globigerinoides, Globorotaloides, Protentella, and Candorbulina are less common but provide bases for tentative correlation with the tropical zones. The "lower Miocene" Zemorrian and Saucesian stages are correlated with the Globigerina sellii through Turborotalia kugleri zones (Oligocene to Aquitanian); the "middle Miocene" Relizian and Luisian stages correlate with the Catapsydrax dissimilis through Turborotalia fohsi zones (Aquitanian to Burdigalian); and the "upper Miocene" Mohnian and Delmontian stages correlate with the Globorotalia lobata through Sphaeroidinella seminula zones (Burdigalian to Sarmatian).

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