Collections from about 600 localities in Micronesia, made by Doctors Tayama, Yabe, and Asano, and by Messrs. Kodaira and Sugawara and the writer, are the basis of this study of the larger Foraminifera found in the Priabonian, Aquitanian, Burdigalian, Pliocene?, and Pleistocene sediments. Seventy-seven species and varieties are described and illustrated. The new species are Kanakaia marianensis, Acervulina (Ladoronia) vermicularis, Biplanispira mirabilis (Umbgrove) forma depressa, B. mirabilis forma elliptica, B. inflata, Borelis parvulus, Gypsina saipanensis G. marianensis, Lepidocyclina (Eulepidina) rotaensis, Discocycline (Discocyclina) indopacifica, and Miogypsinoides saipanensis. The first two are the genotype and subgenotype of the new genus Kanakaia and new subgenus Acervulina (Ladoronia).
The age of the formations of the Mariana Islands and the ecological conditions are inferred from the faunas and lithologic character. The Priabonian and Aquitanian formations with larger Foraminifera were deposited in biohermal, lagoonal, and fore-reef detrital facies of coral reefs. The Globigerina rocks of these horizons are ascribed to comparatively shallow water, but not to eupelagic conditions. The Pleistocene limestones were deposited partly as bioherms and partly as fore-reef detrrital facies.
In the review of the family Nummulitidae, the genus Ranikothalia Caudri is suppressed as a synonym of the genus Miscellanea Pfender. All the species of Nummulites cordelées, of Cizancourt belong to Miscellanea Pfender. Miscellanea is characteristic of the Paleocene.
In the review of the family Miogypsinidae, Miogypsinella Hanzawa is suppressed as a synonym of Miogypsinoides Yabe and Hanzawa. Miogypsinopsis Hanzawa, Miogypsinita Drooger, and Heterosteginoides Cushman are suppressed as synonyms of Miogypsina Sacco. The juvenile features of every species of Miogypsinoides are constant and are used as the prime specific characters.