Introduction. pp. 1. Terminology of the plates of sessile barnacles. pp. 2,3. Material examined. pp. 9. Distribution of American Balanomorph barnacles. pp. 10. Notes on fossil barnacles. pp. 11. Classification. pp. 12. Descriptions of genera and species. pp. 14. Appendix. pp. 337. Explanation of plates. pp. 339. Index. pp. 359-366. This work is a continuation of the author's report on the Cirripedia contained in the United States National Museum, of which the portion relating to pedunculate forms was published in 1907. It was at first intended to record the species of sessile Cirripedia in the Museum, with their localities, and to describe and figure new forms. As the collection was worked over, its wealth in American barnacles became apparent and contains nealy every known species of the Western Hemisphere, many of which show variations special to American waters, altering the scope of this work which may now be described as a monograph of American sessile barnacles.