Two new Mesodictyopsis species, M. akitaensis sp nov and M. miyatanus sp nov., from a Late Miocene to Pliocene freshwater sediment, Japan

摘要

Two new Mesodicryopsis species, M. akitaensis sp. nov. and M. miyatanus sp. nov., are described from freshwater sediment of the Late Miocene to Pliocene Miyata Formation, northern Honshu, Japan. The species are characterized by small size, veluma on inner side of areolae, one fultoportula (sometimes two in M. akitaensis) on central valve face with two (in rare cases three in M. miyatanus) satellite pores, mantle fultoportulae with three satellite pores located on Upper mantle close to valve face/mantle junction. Single rimoportula located in a row of areolae on marginal valve face near valve face/mantle junction in M. akitaensis or valve face/mantle junction on a strip in M. miyatanus. The veluma of M. akitaensis are cribra in inner view and sometimes volae on marginal valve face in outer view while those of M. miyatanus are cribra in both inner and outer view. The two species are compared with two similar reported Mesodictyopsis taxa.