Morphology and ethology of the Middle-Late Jurassic oyster "Liostrea" roemeri (Quenstedt) from the Upper Callovian of the Volga Region near Saratov and Middle Volgian of the Subpolar Urals are considered. The inverted shell shape and presence of a beaked umbo on the right valves allow the assignment of these oysters to a new monotypic genus, Argutostrea Kosenko, gen. nov. Features of the internal valve structure examined in only one specimen suggest that the new genus should tentatively be included in the subfamily Pycnodonteinae Stenzel, 1959 of the family Gryphaeidae Vialov, 1936. It is shown that shell morphology of the oyster A. roemeri is connected with transition to dwelling on floating ammonites.
