1. Department of Operative Dentistry, Showa University School of Dentistry, 2-1-1 Kitasenzoku, Ohta-ward, Tokyo 145, Japan
2. Department of Dental Materials, Nihon University School of Dentistry at Matsudo, 2-870 Sakaecho-nishi, Matsudo City, Chiba 271, Japan
3. Department of Oral Anatomy, Showa University School of Dentistry, 1-5-8 Hatanodai, Shinagawa-ward, Tokyo 142, Japan