Description: Shell large for the genus, whorls 8; nuclear whorls 2, the first smooth and flat-topped, the second with about 30 axial riblets; a noded spiral carina appears on the last 1/3 of the second nuclear whorl; early postnuclear whorls with 9 rounded, sharp edged, antisigmoid ribs decreasing to 7-8 on the body whorl; ribs centrally noded with an intersecting weak spiral keel; other spiral sculpture of distant incised lines, except on the base where there are 8 cords; suture an undulate groove; aperture elongate; columella smooth; outer lip varicose, smooth within except for a single denticle just abapical to the wide, uncallused, unconstricted anal notch; anterior canal short, open; color of 3 distinct bands, a buff band on the upper portion of the whorl, a white band centrally and a light chocolate brown band above the suture, and the base of varying shades of buff. Dimensions of holotype: height 7.8 mm, diameter 3.2 mm. Type Locality: Holotype and 12 paratypes from under rocks, Agua de Chale, Baja California, 30°42'N, 114°41' W. All but 1 are crab specimens. Leg. Helen DuShane, May 1962 and May 1966.
Discussion: Pyrgocythara emersoni is one of the largest species within the genus. P. entersoni seems closest to P. melita (Dall, 1919). melita is smaller and thinner, has more pronounced spiral sculpture, and has purple mark¬ings. It is my pleasure to dedicate this lovely little shell to Dr. William K. Emerson of the American Museum of Natural History, New York.