In all aspects of this work I was assisted and encouraged by a great number of persons. In particular I wish to acknowledge my indebtedness to my supervisor, Prof. Kazuro Hanihara of the Department of Anthropology, Faculty of Science, the University of Tokyo, who guided me at various stages and constructively criticized my work. I also wish to express my gratitude to Dr. Bin Yamaguchi and Dr. Hajime Sakura, Director and Senior Curator, respectively, of the Department of Anthropology, National Science Museum, Tokyo, for their invaluable advice and criticism of this paper.

To Emeritus Prof. Tadahiro Ooë of the Department of Anatomy, Faculty of Medi cine, the University of Tokyo, and Prof. Toshiro Kamiya of the College of Medical Technology and Nursing, University of Tsukuba, Ibaraki, I wish to express my grateful thanks for their permission allowing access to the dental casts of twins under their juris diction. Furthermore, I am highly indebted to Prof, Keiichi Omoto and Associate Prof. Banri Endo of the Department of Anthropology, Faculty of Science, the Uni versity of Tokyo, to Associate Prof. Naohiko Inoue of the Oral Surgery Division, University Branch Hospital, the University of Tokyo, to Prof. Akio Yamauchi of the Department of Anatomy, Faculty of Medicine, the University of Tokyo, and to Asso ciate Prof. Takeru Akazawa of the Department of Anthropology and Prehistory, the University Museum, the University of Tokyo, for their detailed reading and many suggestions in the course of preparation of the manuscript. I would also like to thank Associate Prof. Sadao Shimaguchi of the Department of Anatomy, School of Medicine, Kitasato University, as he kindly permitted me to use his program ADBMS1 in preparing this paper. Finally, my appreciation is expressed to Miss Akiko Nakatsuka for her excellent technical assistance.