Tertiarius kabutoiwaensis sp. nov. and T. minimus sp. nov. from a Pliocene deposit of the Kabutoiwa Formation, central Japan

Abstract

Two new Tertiarius taxa, proposed as T. kabutoiwaensis sp. nov. and T. minimus sp. nov., were described from a freshwater Pliocene crater lake deposit in the Kabutoiwa Formation, Gunma Prefecture, central Japan. The two new taxa have a single rimoportula associated with a mantle fultoportula-bearing costa, a charecteristics of the genus Tertiarius. In addition, T. kabutoiwaensis is characterized by radial areolae rows on valve face, one valve face fultoportula, mantle with a total of 5–15 costae each having a mantle fultoportula. T. minimus is characterized by its very small size, slightly elliptical openings of areolae on marginal valve face and mantle, one valve face fultoportula and a total of 3–6 costae, each with a mantle fultoportula. These two new species are compared to Tertiarius oitaensis from the Omoto Formation, Oita Prefecture and T. satsumaensis from the Koriyama Formation, Kagoshima Prefecture, southwestern Japan. All four Tertiarius taxa, as well as Cyclotella notata from the Omoto Formation are shown herein.