Examination of the type materials ofNavicula subtilissima Cleve (Bacillariophyceae)

Navicula subtilissima was described by Cleve as a new species from ‘a dry rivulet at the shore of Imandra, at the foot of Chibinä, Lapland’. However, the examination of the type slides and the unmounted original material using transmission and scanning electron microscopy revealed that there are at least three finely striated similar taxa distinguishable at the specific level. From these taxa, application of the nameN. subtilissima Cleve is established by selecting a lectotype; a second one is described as a new species,N. parasubtilissima, and the third is identified asN. subtilissima var.micropunctata Germain. The last one warrants a specific status by its fine structural differences in having continuous striae consisting of a single elongate areola occluded by a hymen with perforations arranging hexagonally. The occurrence of these three species is shown in Japanese collections.