New and interesting diatoms from Tibet. III. Valve ultrastructure of two Neidium Pfitzer species from Tibet: comparison with other morphological groups of Neidium and consideration of phylogenetic relationships within the genus

We present light and scanning electron microscope observations on two Neidium species, N. didelta and N. rectum, described by Friedrich Hustedt from Tibet. Neidium didelta and N. rectum have external proximal raphe ends that are deflected in opposite directions, as well as the presence of lacinia and renilimbi, all features of most members of the genus. Both species have a unique combination of characters. Internally, they have coverings over the longitudinal canals and areolae that are pluriseriate, much like the situation in the genus Neidiomorpha, and unlike other Neidium taxa. The areolae at the valve margin are larger than the other areolae comprising the striae. We compare these species with taxa described by Zhu & Chen, and suggest that these taxa are later synonyms of Hustedt's species. We discuss the possible systematic affinities of these taxa within the Neidium and Neidiomorpha lineage.