In the course of studies on Devonian radiolarian faunas from Nanning, Guangxi, SW China (Luo et a1., 1997), numerous smooth siliceous microspheres (Fig. 1) are encountered together with abundant spherical entactiniid radiolarians. The present investigation reveals that these microspheres are siliceous casts formed through precipitation of silica between the medullary and cortical shells of entactiniid radiolarians. As these microspheres are commonly the most abundant particles encountered in some Devonian samples, an understanding of their origins is desirable.

Examined material is from two bedded chert samples collected from the upper part of the Tanhe Formation at Wuxiangling, 5...

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