Nitzschia amphibia var. frauenfeldii has finer striae and fibulae. Denticula kuetzingii and Denticula kuetzingii var. rumrichae have valves that are more linear and not constricted in the center. In addition, Denticula kuetzingii has higher stria density (13-20 in 10µm) and fibula density (5-8 10 µm). Denticula kuetzingii var. rumrichae overlaps in length, but has a differing fibula structure.
BioData Reference
Krammer and Lange-Bertalot 2/2 Pl 100 Fig 18-22
BioData Reference
Krammer and Lange-Bertalot 2/2 Pl 78 Fig 25-26