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TAXONOMY OF THE MARINE DIATOM RESTING SPORE GENERA DICLADIA EHRENBERG, MONOCLADIA GEN. NOV. AND SYNDENDRIUM EHRENBERG AND THEIR STRATIGRAPHIC SIGNIFICANCE IN MIOCENE STRATA

The morphology and taxonomy of the fossil diatom resting spore genera Dicladia Ehrenberg, Monocladia Suto gen. nov. and Syndendrium Ehrenberg in samples of Neogene age from DSDP Hole 438 from the northwest Pacific Ocean were examined. Dicladia is characterized by an epivalve with two conical elevations, and includes three species: D. capreola Ehrenberg, D. mitra Bailey and D. japonica Pantocsek. The new genus Monocladia is established here to receive two new species: M. humilis Suto and M. alta Suto. Monocladia differs from Dicladia by having only one elevation and a single hyaline, branching process on the epivalve. Syndendrium includes two species: S. diadema Ehrenberg and the new species S. akibae Suto. This genus is characterised by two or more hyaline, branching processes that arise from the central portion of the epivalve. Monocladia humilis occurs sporadically in the interval from the lower Denticulopsis lauta Zone (NPD 4A) through the upper Thalassiosira yabei Zone (NPD 5C). Monocladia alta and S. akibae are both characterised by a short-ranging occurrence from the lower Denticulopsis praedimorpha Zone (NPD 5B) through the upper Denticulopsis dimorpha Zone (NPD 5D). Monocladia humilis, M. alta and S. akibae are potentially useful biostrati-graphic markers to improve resolution of Neogene diatom biostratigraphy of the North Pacific Ocean.