Species of the neomenioid aplacophoran genus Helicoradomenia Scheltema & Kuzirian are found only in areas of hydrothermal vents, oceanic ridges, and back-arc basins and have been collected widely in the East Pacific, Southwest Pacific, and the Triple Junction in the Indian Ocean, but not in the Atlantic. As with other vent taxa, species diversity of Helicoradomenia in the East Pacific is greatest south of the subduction zone of the Pacific Plate under the North American Plate, which divided the ridge system into two sectors during the Eocene: the northern Juan de Fuca system and southern East Pacific Rise. Diversity of Helicoradomenia species reflects, in smaller numbers, the diversity of the vent limpets in the two sectors. Two Helicoradomenia sister species are illustrated with the characters that separate them, H. juani Scheltema and Kuzirian (1991) from the northern sector and H. acredema Scheltema (2000) from the southern. They apparently speciated when the vicariant event of subduction of a once continuous ridge occurred.