I. Introduction. II. Description of new cases, by families. 1. M. family. 6. T. family. 11. Pa. family. 2. R. family. 7. S. family. 12. Ka. family. 3. A. family. 8. St. family. 13. Kr. family. 4. L. family. 9. G. family. 14. D. B. 5. K. family. 10. P. family. 15. I. L. III. Types of radio-ulnar synostosis. IV. Phylogenetic and comparative history of radius and ulna. V. Ontogenetic interpretation of radio-ulnar synostosis in man. VI. Bony defects associated with synostosis. VII. Heredity. 1. Family distribution of synostosis. 2. Hypothesis. 3. The criterion of symmetry. 4. Consanguineous marriages. VIII. Conclusion. IX. Summary. I. INTRODUCTION Radio-ulnar synostosis, or congenital synostosis of the proximal ends of the radius and ulna, also sometimes called "congenital pronation," is a condition rarely found in one or both forearms of which the most obvious symptom is an inability to hold the hand supine; i. e.,