1. Two cases of an unrecognised congenital defect of the humeral head are described and the cause is discussed. 2. Only six cases with similar radiographic appearances could be collected from the literature. In most of these cases other skeletal deformities were present, whereas in those now reported only the shoulder was affected. 3. Examination of radiographs suggests that the main deformity consists of lack of development of the capital epiphysis of the humerus. 4. Consideration of the cases, together with experimental data from the studies of Fell and Canti, suggests that the time in development at which the fault occurred was the presumptive joint stage, just when the articular rudiments had separated. 5. A "nociferous agent," acting only for a limited period, and only on certain tissues, is postulated. 6. It is suggested that the defects recorded should be recognised as a group of congenital deformities of the shoulder joint.
