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Spongoplegma variabile Nakaseko, 1971

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Spongoplegma variabilium
Description:—Cortical shell large, spherical, with a rough surface; its shell consists of a single lattice plate, but has a spongy frameworks in very small area; pores of the cortical shell usually well spaced, circular or subcircular, irregular in size and shape, about 15—20 across diameter; its bars vary in thickness, bearing very short thorns on each nodal points; wall of the shell fairly thick. Numerous radial beams connect the cortical shell to a small and subspherical medullary shell, become forked and compose by communication of lateral branches; its texture of the inner shell similar to that of secondary medullary shell at a glance. The medullary shell present as Nigrini pointed out, small in size, subspherical; its pores small, well spaced, hexagonal or subhexagonal, similar in size, with thin bars.
Measurements:— Diameter of the shell, 170—250µ
Remarks: — Spongoplegma variablium is the most frequently found and abundant radiolarians in the middle Miocene formations of Japan, and is the represented species of the Ta—Ar assemblage. Also, it is found commonly in the Nishiyama and Tobetsu formation (Pliocene). This species is similar to Spongoplegma antarcticum HAECKEL in the internal texture of the shell. It is distinguishable from the latter that the cortical shell consists of a single plate, that has a spongy meshwork in very small area, and that the diameter of the shell is smaller.
Nakaseko 1971


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