| The type materials of Pleurosigma formosum W. Smith and P. decorum W. Smith were examined. Both species were separated hitherto by size range, valve shape and stria density based on light microscopy (LM) analysis. However, the type materials show some overlap in the size and stria density ranges, with subtle differences in the oblique stria intersection angle, which is somewhat smaller for P. decorum. Scanning electron microscopy (SEM) analysis shows that both species share several ultrastructural features such as a thick saddle-shaped central raphe nodule, very long overlapping central raphe fissures, short hook-shaped terminal raphe fissures, general morphology of the internal hymenoccluded pores crossed by a recessed bar and presence of some scattered... |
