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Thecosphaera miocenica Nakaseko, 1955

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Thecosphaera (Thecosphaera) miocenica
Description .—Shell entirely globular, spiny, and with three concentric spheres in the ratio of 0.27, and 0.12 diameter of the outer shell respectively ; cortical shell with moderately thick wall, surface with about 16—18 pores across the diameter ; these pores fairly small, circular or subcircular, more or less uniform in size, deeply set, and with the subhexagonal frames; surface of the cortical shell spiny, its spines extremely short, arising at each node of the subhexagonal meshworks ; secondary medullary shell with about seven pores across the diameter, its pores circular, uniform in size, well spaced, and thick bars connected with the cortical shell by means of six beams ; primary medullary shell very small.
Measurement of the illustrated specimen :—Diameter of the cortical shell, 138µ ; of the secondary medullary shell, 36µ ; of the primary medullary shell, 16µ ; of the pores, 6-8µ ; thickness of the wall, 12µ.
Remarks :—Thecosphaera californica Clark and Campbell (1942, p. 22, pl. 4, fig. 7) of the Eocene of California to which the present species resembles, has not the hexagonal frames and therefore, should belong to the other sub-genus (Thecosphaerella). But these two species have many common features, so the present writer considers that they are closely related to each other. They differ from T. favosa Haeckel (1887, p. 79) recorded from the Central Pacific, Station 268 and the Tertiary rocks of Barbados in that the ratio of the three spheres is 1:2:9, in that pores are subcircular or circular and less in number, and in that the diameter of the shell is smaller.
Localities :—A 2, Joyama mudstone ; B 1, Nishidojima mudstone.

Nakaseko 1955
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.
Nakaseko 1971


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