Cassid gastropods are useful in delineating molluscan zones of the late Paleogene and early Neogene and in correlating between isolated basins of marine strata in Oregon, Washington, Alaska, and that part of California north of the San Andreas fault. Three middle Tertiary cassid species of the genus Liraccassis occur in the Pacific Northwest: L. rex, L. apta, and L. durhami. Liracassis rex is older than L. apta and is restricted to the Liracassis ["Echinophoria"] rex Molluscan Zone, which includes the type section of the Blakeley Formation of Weaver (1912) in Washington, and is equivalent to the early Zemorrian Foraminiferal Stage, which is assigned to the early Oligocene. Liracassis apta is restricted to the Liracassis ["Echinophoria"] apta Molluscan Zone; the molluscan zone is equivalent to the late Zemorrian and early Saucesian Foraminiferal Stages, currently assigned to the late Oligocene and early Miocene, respectively. Liracassis durhami occurs with both L. apta and L. rex, and in the past, because L. durhami is difficult to distinguish from the other species, it has caused confusion in recognizing the molluscan zones.--Modified journal abstract.