The new gonyaulacacean dinoflagellate cyst genus Nidarocysta is described from the Oxfordian/Kimmeridgian boundary oi Mid-Norway and East Greenland. The new genus is related to Leptodinium KLEMENT 1960 emend. STOVER & EVITT 1978, but differs in having an adnate operculum (3 ＇＇), a very unusual archeopyle opening mode, and by the absence of ＇＇true＇＇ parasutural septa. The new morphological term ＇＇random splitting＇＇ is introduced for this unusual opening mode. Based on the peculiar combination oi some morphological features, the possible affinities of this new genus with non-marine dinoflagellate cysts are discussed. Nidarocysta jubilaea gen. et sp. nov. is presently regarded as an useful marker for the latest (possibly Late) Oxfordian-earliest Kimmeridgian of the European boreal province (Norwegian Sea, Greenland, British Isles and Denmark).