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Two new Mesodictyopsis 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. [ABSTRACT FROM AUTHOR]

The first record of the genus Pliocaenicus from middle Miocene deposits is presented in this paper. This record from Changbai (Jilin Province, China) places the origin of the genus further back into themiddleMiocene. The genus has been so far reported from the late Miocene/early Pliocene till Recent. The population from Changbai belongs to the group of Pliocaenicus species possessing complex alveolae, such as P. cathayanus Wang, P. jilinensis Wang and P. omarensis (Kuptsova) Stachura-S. et Khursevich. Both P. cathayanus and P. jilinensis have been described from China (Pliocene deposits), and are only known from the type locality. The third species, P. omarensis is reported from Eurasia and Africa (age range: late Miocene-Pleistocene). In this report we describe P. changbaiense, a new species from China, and focus on those characters having the potential for developing further an evolutionary and taxonomical concept in Pliocaenicus. We anticipate that these will contribute to our understanding of the driving forces of diatom dispersal. [ABSTRACT FROM AUTHOR]