Valve Morphology and Ultrastructure of the Enigmatic Diatom Cymbopleura semirecta (Manguin ex Kociolek & Reviers) Bahls

We report the occurrence of the rare and enigmatic diatom species Cymbopleura semirecta (Manguin ex Kociolek & Revi-ers) Bahls here for the first time from the southern Rocky Mountains. We review the current systematics and historic taxo-nomic placement of this taxon and its placement in both Cymbella and Cymbopleura. Valve morphology and ultrastructure are presented based on light (LM) and scanning electron microscope (SEM)_investigations. LM and SEM observations show this species lacks apical pore fields, a key feature helping to definitively place this taxon in Cymbopleura. We discuss the distribution of this species in higher latitude freshwater environments.