Specimens (in FH and PC) of Cumathamnion sympodophyllum M.J. Wynne and K. Daniels (Delesseriaceae, Rhodophyta) made by C. G. Pringle at Cape Mendocino, northern California in 1882 are the first known collections of this rare monospecific genus. A collection (US) made by E.Y. Dawson from Trinidad Head, Humboldt County, and reported in 1965 as a “very compact, short-bladed” form of Delesseria decipiens J. Agardh is re-determined to be C. sympodophyllum. Records of C. sympodophyllum from Oregon, Washington, and British Columbia (Canada) are based on one collection from British Columbia, which cannot be verified. This taxon is most likely restricted to very exposed lower littoral rocky outcrops of Sonoma, Mendocino, and Humboldt counties in California.