Malignant tumors arising from the periosteum or from structures immediately adjacent to the periosteum and growing out away from the bone without erosion of the bone cortex or invasion of the medullary cavity have been described by Ewing1 and Codman.2 At present these tumors are included among the bone tumors in the classification adopted by the Sarcoma Committee of the American College of Surgeons under the heading of periosteal fibrosarcoma. Since the publication of the classification of the Bone Sarcoma Committee much discussion has arisen as to whether these tumors rightly belong with bone tumors, because they seem to have no tendency to involve the bone and because there is no proof that they arise from bone structures. My purpose in this article is to report two cases of benign tumor which seem to bear the same relationship to bone in that they seem to have developed from