Placoneis Mereschkowsky: The re-evaluation of a diatom genus originally characterized by its chloroplast type

Several freshwater naviculoid diatoms, for which Mereschkowsky erected the genus Placoneis, have been re-examined with light and electron microscopy. The findings reveal that, in addition to their distinctive chloroplast type, their valvar structure also differs significantly from that of Navicula sensu stricto, and their separation from the latter is warranted. Placoneis is therefore re-erected to include them and an emended description and generitype are given. Some species included by Mereschkowsky have however been excluded, while other, more recently described taxa have been transferred to Placoneis. The position of a number of species remains unclear. Some of the taxonomic and nomenclatural problems associated with this group are briefly discussed