• diatom;
  • Ehrenberg;
  • Navicula abiskoensis;
  • Navicula amphibola;
  • Navicula semen;
  • taxon delimitation;
  • type collections

The identity of three naviculoid diatoms originally described by Ehrenberg and now placed in the genus Placoneis was investigated. Type specimens and material from both the Ehrenberg Collection (Berlin) and the diatom collection at the Natural History Museum, London, were examined with LM and also SEM where possible. The concept of each species had changed since Ehrenberg named them. The current understanding of Placoneis gastrum and Navicula semen can be traced back to Donkin, and that of Placoneis placentula to Grunow. LM and SEM studies of N. semen support its transfer to Placoneis. It is proposed that the specific epithets gastrum and placentula should be conserved with conserved types to maintain established practice, and the name P. navicularis applied to N. semen sensu Donkin. Type material of Placoneis elginensis and Placoneis anglica was re-examined from slides in the Natural History Museum and it was concluded that P. elginensis has been incorrectly applied to specimens of a second species, for which the name P. anglica was appropriate. The delimitation of these species, P. pseudanglica, and others previously included in P. elginensis is reviewed. LM and SEM observations of Navicula abiskoensis and Navicula amphibola support their transfer to Placoneis. Six new combinations are made for additional taxa belonging to Placoneis: P. constans, P. insignita, P. interglacialis, P. porifera, P. rostrata and P. subgastriformis. The systematic position of the genus Placoneis is reviewed. A revised generic description and a summary of the species currently assigned to Placoneis are included. © 2003 The Linnean Society of London, Botanical Journal of the Linnean Society, 2003, 141, 53–83.