Neidiomorpha gen. nov. (Bacillariophyta): A new freshwater diatom genus separated from Neidium Pfitzer

Comparative morphological (including plastids) and ultrastructural (frustule) analyses of numerous taxa within the established diatom genus Neidium Pfitzer led to the conclusion that N. binodiforme Krammer and N. binodis (Ehrenberg) Hustedt can be identified as striking exceptions. They would be more coherently allocated in a new freshwater benthic diatom genus, Neidiomorpha gen. nov. This proposed genus differs from Neidium mainly by the longitudinal structures (shallow "caves" in the mantle instead of distinct canals at the junction between valve face and mantle), by areolation pattern (including the absence of renilimbi on the internal valve surface), and raphe endings (no silica flaps causing a bifurcate appearance of the polar endings and no central endings deflected towards opposing directions). Chloroplast number and arrangement (two valve- and girdle-appressed chloroplasts placed fore and aft in the two halves of the cell), overall valve outline, and ecology (preference for medium to high conductivity freshwaters) are likely to be further relevant distinctive elements. Observations on the distribution, and habitat and choriotope preference of the two species combined with the new genus are also provided.