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Diatoms from the type section of the Luisian Stage in central California correlate with the lowermost part of the Denticulopsis lauta Zone through the lower part of subzone "a" of the Denticulopsis hustedtii –D. lauta Zone and are early Middle Miocene in age (about 16.0 to 14.0 Ma}. Rocks assigned to the Luisian Stage by benthic foraminifers elsewhere in California exhibit little diachroneity in terms of diatom biostratigraphy; however, detailed studies of boundaries have not been done. Planktic diatoms dominate the assemblages, although an increase in benthic and tychopelagic diatoms in the overlying Hames Member of the Monterey Formation probably reflects shoaling. A cooling trend is suggested by diatom assemblages n the upper part of the section and is most marked near the top of the type Luisian. This cooling trend is supported by data elsewhere in the Pacific Basin as well as by both megafossil and microfossil studies in California. One hundred four diatom taxa are documented, and one new species, Coscinodiscus volutus Baldauf, is described.