Several months ago, when examining the brain of a patient who had exhibited an unusual and interesting clinical picture, one of us discovered a tumor extending throughout the ventricular system which was quite unlike any of the growths commonly seen in the cerebral ventricles. It was recalled that three similar cases had been observed in this clinic within approximately ten years. On consultation of the records, it was found that the clinical picture in these cases presented many features in common. In two of the older cases the histologic character of the tumor was, indeed, identical with that of the tumor under investigation. In the third case, which would have made a total of four cases, the tissues had been lost, and the descriptions of the sections were scarcely adequate to establish the nature of the tumor. This case has, therefore, been omitted. The three tumors which are the subject