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Indian Ocean. Biostratigraphy and paleoceanographic implications of the differences in assemblage compositions are discussed. The assemblage analysis for the middle and late Eocene at Sites 356 and 357 in the mid-latitude southwest Atlantic provides insight into the paleolatitudinal distribution of diatom species. In the middle Eocene the diatom assemblages at these sites are composed of low-latitude and cosmopolitan species, whereas during the late Oligocene high-latitude species are present, reflecting an extension toward the equator of the circum-Antarctic assemblage that can be related to climatic cooling and probably intensified meridional wind and surface water circulation. In addition, the Eocene-Oligocene planktonic diatom zonation developed in the low-latitude Atlantic and the Caribbean (Fenner, in press) is tested against the moderately to well-preserved early Oligocene diatom assemblages of the Pacific sites (except for Site 292, at which preservation of diatoms was too poor), and those of middle and late Eocene age at DSDP Site 167. The age assignments obtained by applying this stratigraphic zonation are in accordance with those obtained using other planktonic microfossil groups, thus proving the applicability of this zonation. The wide occurrence of the same species through the equatorial Atlantic and Pacific oceans and the identical sequence of first appearances have both to be seen as a result of the homogenizing effect of unobstructed circum-equatorial circulation in Tethys. In the diatomaceous Eocene-Oligocene section of Site 167 in the equatorial Pacific, the change in assemblage composition toward an assemblage dominated by Cestodiscus spp. in the early Oligocene occurs at about the Eocene/Oligocene boundary, as it also does at DSDP Site 366 in the equatorial Atlantic. The value of this change in assemblage composition as a paleoclimatic indicator and stratigraphic marker is suggested. The only Indian Ocean sites (Sites 216, 217, 220) which contain Eocene-Oligocene diatoms were located just south of the equator during the Eocene. But preservation of diatoms is too poor to allow stratigraphic assignments or to discuss assemblage compositions in relation to other known sections.

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