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The diatom assemblages described in this study are from deposits at Sites 494, 495, 496, and 499, located within the area of the Middle America Trench, Guatemala transect. Hole 495 was drilled on the Cocos Plate, on the seaward slope of the Middle America Trench; Hole 499 was drilled directly on the trench bottom; and Holes 494 and 496 were drilled in the lower and middle parts of the Guatemala continental slope. This transect of sites across the trench enables us to correlate deposits from the oceanic plate to the continental margin. In general, we investigated the diatoms found in Pleistocene and Pliocene sections of all the sites. Our investigation of diatoms of Site 495 was broader and included diatoms from the Pleistocene to the lower Miocene— our aim being to obtain comprehensive information about the composition and distribution of diatoms in the Neogene of the region concerned. The section is unique in its completeness and is essential to this account. All sites located on the trench bottom and on the continental slope penetrated thick beds of Pleistocene sediments (180-200 m); only small portions fell within the lower Pleistocene. Pliocene sediments are poor in diatoms and not thick. On the whole, Pliocene and Pleistocene sediments are characterized by poor diatom remains as a result of terrigenous-volcanogenic dilution. Our analysis of the taxonomy and the ecology of the Pliocene and Pleistocene diatom flora is aimed at establishing the stratigraphy of deposits drilled during Leg 67. Deposits penetrated by drilling during DSDP Leg 67 are stratigraphically subdivided on the basis of L. Burckle's diatom stratigraphy (Burckle, 1972; Burckle and Opdyke, 1977) and modified by the data of subsequent investigations (Kazarina, 1975, 1978; Burckle, Trainer, 1979). The changes introduced into Burckle's (1972) scale are minor and consist of the following: the Nitzschia reinholdii Zone (1.8 m.y.-63O,OOO yr.) in Burckle's scale is replaced by a stratigraphically equivalent zone, the Nitzschia fossilis Zone, and in the Pseudoeunotia doliolus Zone (700,000 yr.-present day) additional units with Coscinodiscus pseudoincertus and those with Coscinodiscus nodulifer were distinguished. In addition to these species and zones, we concentrated on the stratigraphic distribution of some particu-

Distribution of Diatoms in Pliocene and Pleistocene Deposits from the Middle America Trench off Guatemala