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THE FIRST STAGE in this investigation of the microplankton (dinoflagellate cysts and acritarchs) of the Bracklesham Beds of the Isle of Wight has involved the re-examination of the classic Eocene cliff sections at Whitecliff Bay in the east and Alum Bay in the west. Fisher’s concept of the type section of the Bracklesham Beds at Whitecliff Bay (Fisher 1862) formed the basis of the present study, although his interpretation of the lithological divisions has been slightly modified. At Alum Bay the Bagshot and Bracklesham Beds have been redefined. Several breaks have been noted in the sequence, all but one of which are succeeded by a pebble bed. Rich microplankton assemblages have been recovered from the Bracklesham Beds of both sections. Mollusca and Foraminifera are essentially restricted to the marine clays and greensands. Microplankton also occur in these sediments and in the ‘laminated beds’ (Fisher 1862, p. 67). This difference in distribution is particularly significant at Alum Bay where all but the highest 16 metres (52 feet) of the Bracklesham Beds were previously thought to be devoid of marine fossils. The stratigraphical ranges of selected microplankton species have been used to define five microplankton zones in the type section of the Bracklesham Beds at Whitecliff Bay. All five zones have been recognized in the redefined Bracklesham Beds at Alum Bay, and the divisions within the Bracklesham Beds of both sections have been correlated for the first time. As a result of this correlation the following conclusions were reached concerning the section