This study has been supported by many cooperative skilled divers and other kind persons in Okinawa and Miyako Islands. We sincerely thanks Messrs. Shuichi Ohashi, Shigemitsu Kinjo, Nagaaki Yosa, Hiroyuki Gibo, Hiroyuki Kinjo and Kiyoshi Kijima and Miss Miyoko Uchima of Okinawa, Messrs. Mitsutoshi Taniguchi and Ayato Tamura of Miyako, and Mr, Shinichi Shimizu of Tokyo for their self sacrificing contribution in collecting samples. Some comparative samples outside the Ryukyu Islands were supplied by Messrs. Peter Reber, Toshio Kanazawa and Hisamichi Nabeshima. Mr. Sueo Kaneko kindly gave us full information about his unpublished data on the Philobryidae and Propeamussiidae. Concerning the taxo- nomic identification, one of us (Hayami) received various important suggestions and comments from Professor John Alien (Scotland), Drs. Thomas Waller, Eugene Coan, Paul Scott and Robert Prezant (U. S. A.) and Dr. Jean Poutiers (France) at the time of the 1991 International Marine Bivalve Symposium at Berkeley. Our cordial thanks are due to Dr. R. K. Dell (New Zealand) and Dr. Philippe Bouchet (France) for their instructive information in relation to the present study, to the staff members of the Australian Museum for their kind loan of some type specimens, to Drs. Tadashige Habe and Akihiko Matsukuma and Mr. Eiji Tsuchida for their kind assistance in observing comparative specimens and literature, to Dr. Yasunari Shigeta and Mr. Takao Ubukata for their assistance in scanning electron microscopy, and also to many faculty members and students of the Paleobiological Laboratory of the University of Tokyo for their fruitful discussions. Among others, Professor Kazushige Tanabe kindly reviewed the final manuscript and gave us important comments and suggestions. This study was financially supported by grants from the Ministry of Education, Science and Culture, Japan (Nos. 01460062, 02402017, 02640616, 04454032), the Fujiwara Natural History Foundation, and Research Institute of Marine Invertebrates.