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Thecosphaera miocenica Nakaseko, 1955

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Thecosphaera miocenica
Description : —Shell entirely globular, with rather smooth surface, and with three concentric spheres; outer shell with moderately thick wall, surface with about 18—20 pores across the diameter; its pores fairly small, circular, regular in arrangement, uniform in size and shape, and with hexagonal frames; secondary inner shell about 0.3 of the outer shell in diameter, with about 8 pores across the diameter; its pores circular, uniform in size, well-spaced, and with thin bars; primary inner shell very small, about 0.15 of the outer shell in diameter; three spheres connected by radial beams, its beams thin, rodlike, about 10 in number.
Measurements:-Diameter of the outer shell, 130-170µ; of the secondary inner shell, 35-45µ; of the primary inner shell, 15-20µ.
Remarks:-The species was reported by the writer from the Joyama formation (middle Miocene) in Toyama Prefecture. It occurs abundantly in the Japanese Miocene formations, and is the characteristic or represented species of the Ta-Ar assemblage. This species is similar in having inner shell noticeably smaller than outer shell to Thecosphaera californica which was recorded by Campbell and Clark from the Eocene formation of California, but differs from the latter in having smaller outer shell and hexagonal frames. It is distinguishable from Thecosphaera japonica as described under that species by that the pores of the outer shell is larger in number, and the diameter of the inner shell is smaller.











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