FINE MORPHOLOGY OF COSCINODISCUS JONESIANUS AND COSCINODISCUS CQMMUTATUS AND THEIR TRANSFER TO COSCINODISCOPSIS GEN. NOV.

The present study was devoted to the morphology, taxonomy, and distribution of Coscinodiscus jonesianus (Greville) Ostenfeld and C. commutatus Grunow. As far as was determined there were no previous studies of the fine morphology of the latter taxon. Therefore, this analysis presents differences between the taxa and reliable information for biogeographical studies. Planktonic samples were collected between October 1994 and September 2000 at seven stations along the northern coast of Buenos Aires Province. Samples and slides from the Hustedt Collection containing C. commutatus from the North Sea were analyzed and compared with material collected from Buenos Aires coastal waters. Raw and cleaned samples were analyzed using light and scanning electron microscopy. The presence/absence of an irregular ring of mesorimoportulae over the valve surface, and the striation pattern of the valvocopulae were determined as the only striking features that differentiated the taxa. Based on the modern generic limits of Coscinodiscus, C. jonesianus and C commutatus differ from other species in the external morphology of micro-rimoportulae and macrorimoportulae. We therefore propose the creation of the new genus Coscinodiscopsis Sar & Sunesen.

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