A study of planktonic Foraminifera from the Upper Cretaceous of California and northwestern Mexico forms a basis for erecting a preliminary biostratigraphic zonation.

The Praeglobotruncana stephani assemblage zone, of late Cenomanian age, is characterized by species of Rotaliporidae.

Strata of early Turonian age are recognized by the first appearance of bi-keeled globotruncanids which characterize the Globotruncana imbricata assemblage zone. Within this zone occur Praeglobotruncana helvetica, G. kuepperi, and several undescribed species. The Globotruncana coronata-G. inornata assemblage zone includes rocks of late Turonian to early Senonian age and contains distinct species of Globotruncana, Clavihedbergella, and Hedbergella.

The Globotruncana arca assemblage zone, of late Senonian age, contains several important stratigraphic markers, e.g., Globotruncana ventricosa, G. havanensis, G. elevata, and Rugoglobigerina rugosa. These indicate a Campanian to early Maestrichtian age. The planktonic Foraminifera which define the late Maestrichtian in other parts of North America are unrecorded in the eastern Pacific.

The stratigraphic and geographic distributions of selected species of Praeglobotruncana, Globotruncana, and Rugoglobigerina are compared with those from the Atlantic Coast and Gulf-Caribbean area. This comparison suggests that Globotruncana arca, G. linneiana, G. fornicata, and several other species were cosmopolitan, whereas such species as G. calcarata were latitudinally restricted during the Late Cretaceous. The genera Abathomphalus and Plummerita are found exclusively in the Tethyan region. Globotruncana gagnebini, G. subcircumnodifer, and others are reported only from the western margin of the Atlantic Ocean; G. kuepperi, G. churchi, and G. putahensis are unknown outside of the Pacific basin.

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