VOLUME 20 (2)/April 1990

Geological lnstitute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Rljksuniversiteit Utrecht, Laboraroritum voor Paleobotanie en Palynologie, Heidelberglaan 2, Utrecht 2506, Nederland

Geophysical Institute, Bulgarian Acadrmy of Sciences, 1113 Sofia
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Geophysical Institute, Bulgarian Acadrmy of Sciences, 1113 Sofia