Abstract
The centric diatom Polymyxus coronalis was studied using light microscopy and scanning electron microscopy. It was abundant in plankton samples from Babahoyo River, Guayaquil, Ecuador. Slide 893 from the Jacob W. Bailey Collection of the Farlow Herbarium has been designated as the lectotype. This slide contains several frustules of P. coronalis and represents material from the Para River, Brazil, from where it was originally collected. The retention of Polymyxus as a genus separate from Actinoptychus is suggested, at least until the generic characteristics of Actinoptychus are made clear. Comparisons are made with A. senarius and A. splendens, as P. coronalis has been considered by most workers to be a synonym of these species.
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