From its beginnings with the preparation of previous articles, this study has been supported by the generous help of numerous persons and organizations. I would first like to express my sincere thanks to Dr. Thomas R. Waller (Smithsonian Institution) for his kind advice, encouragement and critical reading of the first draft of this manuscript as well as for his hospitality during my stay in Washington, D. C. His unpublished data on the type specimens of early described pectinids in various European institutions, which he was good enough to make available, were particularly instructive for the present study. In the course of this study I received much invaluable information and many suggestions from the following scientists, to whom I am also greatly indebted: Professor Stephen Jay Gould (Harvard University), Professor Richard A. Reyment (Uppsala University), Dr. Motoo Kimura and his colleagues (National Institute of Genetics, Mishima), Professors Tetsuro Hanai and Masuoki Horikoshi (University of Tokyo), Dr. Kiyotaka Chinzei (University of Tokyo), Dr. Tomowo Ozawa (Hyogo Education University), Dr. Toshiyuki Yamaguchi (Chiba University), Dr. Tadashige Habe (Tokai University), Dr. Takashi Okutani (National Science Museum, Tokyo), Dr. Katsura Oyama (Toba Marine Aquarium) and Dr. Henk H. Dijkstra (Sneek, Netherlands). Dr. D. D. Swinbanks (University of Tokyo) kindly read the final draft of this manuscript and gave me many suggestions in English writing. The material studied here was kindly shown, loaned or donated by the following persons, without whose generous cooperation this work could never have been completed: Drs. Norman D. Newell and William K. Emerson (American Museum of Natural History), Dr. Joseph Rosewater (Smithsonian Institution), Dr. Suguru Ohta and Mr. Eiji Tsuchida (University of Tokyo), Dr. Minoru Imajima and Messrs. Akihiko Matsukuma and Tomoki Kase (National Science Museum, Tokyo), Dr. Hiromichi Hirano (Waseda University), the late Professor Tokio Shikama (Yokohama National University), Mr. Yoshiaki Matsushima (Kanagawa Prefectural Museum), Professor Sakae Ohara (Chiba University), Dr. Hirosbi Kitazato (Shizuoka University), Dr. Kenshiro Ogasawara (Tohoku University), Mr. Shoichiro Hayashi (Mikawa-isshiki), Mr, Kazuo Kurohara (Tosa-shimizu), Mr. Atsushi Fujii (Kitakyushu City Museum), Professor Tsugio Shuto and Mr. Yoshio Sato (Kyushu University), Mr. Aizo Yamamoto (Nagasaki), Dr. Hiroyuki Otsuka (Kagoshima University) and Professor Tomohide Nohara (Ryukyu University). I am also much indebted to the staff members of the following institutions for making available various facilities for my study of their collections: Department of Invertebrate Zoology, National Museum of Natural History; Department of Invertebrates, American Museum of Natural History; Institute of Geology and Palaeontology, Tohoku University; Department of Zoology, National Science Museum [Tokyo]; and Geological Institute, Yokohama National University. I thank the captains and crews of R/V Tansei and R/V Rinkai as well as many students of the University of Tokyo for their cooperation in dredging Recent samples, Dr. Michio Shigei and other staff members of the Misaki Marine Biological Station for making available various facilities, and also Mmes. Reiko Mitsuda and Yoko Suwa and Misses Kasumi Ohma, Masae Toyama and Megumi Ikawa for their patient efforts in the preparation of samples, SEM photographs and this manuscript. Expenses for this study were partly defrayed through Grants-inaid (Nos. 348023, 56540480) from the Ministry of Education, Science and Culture, Japan.