Nitzschia radicula is very similar to N. incognita, and further work may show them to be the same species.
Nitzschia alpina and N. perminuta have fibulae that are continuous through the median.
Nitzschia acidoclinata has linear valves.
Nitzschia fonticola, N. frustulum, N. hantzschiana and N. liebetruthii have wider valves and lower stria densities.
BioData Reference
Krammer and Lange-Bertalot 2/2 Pl 68 Fig 1-8