Neoplasms arising from the sheath of the acoustic nerve are not uncommon; they comprise from 8 to 10 per cent of all intracranial tumors. Ordinarily these growths are closely applied to the portion of the nerve extending from the internal auditory meatus to the pons, and frequently there is close attachment of the tumor to the bony wall of the meatus, with erosion of that structure to a variable extent. The case reported here is of unusual interest because the tumor arose completely within the petrous bone and did not present itself to view in the usual location. Rather, it eroded the bone extensively and was disclosed at operation Fig. 1.—Roentgenograms of the skull, indicating (A, Towne position) extensive erosion of the ridge of the right petrous bone and (B, Stenver position) erosion in the region of the right internal acoustic meatus. as a bulging mass on the posterior surface