Internal test morphology of the genus Rectobolivina (Cushman, 1927) from the Late Cenozoic Miyazaki Group, southwestern Japan

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Six species of the genus Rectobolivina (benthic foraminifera), R. asanoi, R. bifrons, R. clavata, R. discontinuosa, R. raphana and R. striatula from the Late Cenozoic Miyazaki Group of southwestern Japan were taxonomically studied, particularly focusing on internal test morphology. Two new species of Rectobolivina, R. clavata and R. discontinuosa, and one new replacement name, Rectobolivina clavatostriatula, are proposed. Scanning electron microscopic and optical microscopic observations of dissected specimens and thin sections reveal that the chamber wall of the genus Rectobolivina is bilamellar, and that the toothplate is an extension of the chamber wall, a twisted plate strongly folded at both edges and connecting successive foramina. The toothplate constantly attaches with one of its sides to the axial side of the inner margin of the preceding foramen. It exhibits a lamellar structure, showing that the extension of the outer lamella of the chamber wall is covered by two thin inner lamellae on both axial and peripheral sides. The six species of Rectobolivina display a clear stratigraphically separated distribution during the period from latest Miocene to earliest Pleistocene.
2002-06-28