Abstract
Pliocaenicus nipponicus sp. nov. is d e scribed from a Pliocene freshwater diatomite of the Kabutoiwa Formation in Gunma Prefecture, central Japan. The species is characterized by wide mantle interstriae with fultoportula openings on the lower part and an external ligula-like segment covering the valvocopula slit. Internally, each costa has a mantle fultoportula, and both mantle fultoportulae and valve face fultoportulae have two satellite pores. The new species is compared to similar species of Pliocaenicus: p costatus (Lupikina)Flower et al. (sensu lato), P. jilinensis G. Wang and P. omarensis (Kuptseva) Round et Håk. Observations of the external ligula-like segment have been included as recent research suggests its usefulness as an identifying characteristic.
