It is a great pleasure to express our sincere thanks to Professor Emeritus Teiichi Kobayashi (Japan Academy) for his continuous encouragement and to Professor Tetsuro Hanai (University of Tokyo) for his constant interest and kind advice about the planning of this work and his reading of the manuscript. We are also much indebted to the following persons: Dr. Ellis L. Yochelson, Dr. Norman F. Sohl and Dr. John Pojeta, Jr. (National Museum of Natural History, Washington, D.C.) for their kind reading of the sections on the Paleozoic Gastropoda, Mesozoic Gastropoda and Paleozoic Bivalvia, respectively; Professor Toshimasa Tanai and other staff members of the Department of Geology and Mineralogy, Hokkaido University, where we studied the type material of the late Professor Takumi Nagao with their permission and assistance; Professor Yoldchi Takayanagi and other staff members of the Institute of Geology and Palaeontology, Tohoku University, where many type specimens could be examined through their courtesy; Professor Keiji Nakazawa (Kyoto University), Professor Emeritus Tatsuro Matsumoto (Kyushu University), Professor Yoshihisa Ota (Fukuoka University of Education), Professor Masafumi Murata (Kumamoto University), Dr. Tamio Nishida (Saga University) and Dr. Katsura Oyama (Geological Survey of Japan) for their useful information and advice; Mr. Takeo Ichikawa and Mrs. Kazuko Hamura for their patient assistance in the curatorial work. Acknowledgments are also due to Professor Nobuo Egami, the Director, and other staff members of this Museum for their arrangement for the publication of this study.