Recently I have had the opportunity of working over a large accumulation of material — mostly from California and adjacent South-western States — belonging to the Geometrid genus Hydriomena; this material was received through the courtesy of Dr. J. Comstock of the Los Angeles Museum and Mr. J. Sperry of Riverside, Calif. I have utilized the opportunity afforded by the identification work on these specimens to study the female genitalia of a number of the species with a view to ascertaining whether good specific characters might be found in these organs, this feature having been entirely neglected in my former revision of the genus in 1917 (B. & McD. Contributions, Vol. IV, No. 1). Surprisingly good results have been obtained and a relationship of the species brought out which supports a contention I have put forward on several occasions, vis., that the present grouping, based simply on palpal length, is largely artificial.