New discoveries of Cenozoic deep-water hydrocarbon seep deposits and continued collecting at previously documented sites in the North Pacific region have resulted in additional fossils of vesicomyid bivalves and necessitate a systematic review. We report five new vesicomyid species, including four species from western Washington State, USA: Isorropodon humptulipsense sp. nov. from middle to upper Eocene strata of the Humptulips Formation and the “Siltstone of unit B”, the oldest record for Isorropodon, Pleurophopsis thieli sp. nov. from upper Eocene to lowermost Oligocene strata of the Lincoln Creek, Makah, and Pysht formations, and Pliocardia? guthrieorum sp. nov. and Squiresica plana sp. nov. from Oligocene strata of the Lincoln Creek and Pysht formations. The new species Squiresica yooni sp. nov. is from the Middle Miocene Duho Formation in South Korea. We report possibly the as-yet oldest Vesicomya from a lower Oligocene seep deposit in the Lincoln Creek Formation in western Washington. Pliocardia kawadai was previously only known from Lower to Middle Miocene strata in Japan; with our new record from the Lower to Middle Miocene Astoria Formation in western Washington, this species represents the first fossil vesicomyid species with a trans-Pacific distribution. The large and elongated Pleurophopsis chinookensis is restricted to upper Eocene strata; previous Oligocene records are shown to belong to other species.

Key words: Bivalvia, chemosymbiosis, deep-sea, Japan, South Korea, USA, Washington.

Steffen Kiel [steffen.kiel@nrm.se; ORCID: https://orcid.org/0000-0001-6281-100X], Swedish Museum of Natural History, Department of Palaeobiology, Box 50007, 10405 Stockholm, Sweden. Kazutaka Amano [amano@juen.ac.jp; ORCID: https://orcid.org/0000-0002-8320-8661], Joetsu University of Education, Joetsu 943-8512, Japan. James L. Goedert [ jamesgoedert@outlook.com; ORCID: https://orcid.org/0000-0002-0257-7255], Burke Museum of Natural History and Culture, University of Washington, Seattle, Washington 98195, USA.