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Abstract
Two new species in the Thalassiosira trifulta group are described from late Pliocene and Pleistocene sediment samples from the northwest Pacific Ocean. Thalassiosira exceptiuncula Shiono sp. nov. and Thalassiosira variantia Shiono sp. nov. possess characteristic features that include the external openings of a central fultoportula, an occluded areola, and several areolae present within a depressed central area. T. exceptiuncula is an unusual species in the T. trifulta group because it possesses small external spines on the submarginal zone. Furthermore, both new species differ from other members of the T. trifulta group in possessing areolae within the depressed central area.