| Summary: | Background and aims – Confusion over the taxonomic identity of Neidium iridis (Ehrenb.) Cleve (= Navicula iridis Ehrenb.) has persisted for more than 130 years with identifications of valve shapes varying from elliptical to linear and sizes from 100 to 300 µm. This confusion can be linked to the rarity of the species and poor attention to the original line drawings of Ehrenberg. This study examines the type material of Navicula iridis and further uses additional material from North America to define the species Neidium iridis.Methods – Mica and material from Christian Gottfried Ehrenberg’s Collection were studied to examine Navicula iridis. Light and electron microscope techniques were used to document the variability of valve morphology of original material in comparison with other fossil and recent materials.Key results – Ehrenberg designated no nomenclatural type for Navicula iridis at the place of its first description, and until recently the original material was not studied. One freshwater extant locality was identified (New York, West Point) in the original 1843 publication, and in his notes and drawings. Examination of the micas and material from the type locality, showed that Navicula iridis was very rare. Two specimens were found that matched the line drawing presented by Ehrenberg. In addition, we examined Ehrenberg’s original material with SEM, as well as two other localities in North America and present LM and SEM documentation of additional valve forms. A further evaluation of the status of Neidium columnaris, Neidium maximum and an unknown taxon in relation to Neidium iridis is presented.Conclusions – The findings of this study elucidate the taxon Navicula iridis (= Neidium iridis) as described by Ehrenberg in 1843. This will assist in the identification of other large Neidium taxa which to date have been associated with Neidium iridis.
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