Nitzschia exilis is distinguished from N. columbiana, with which it co-occurs, by being more slender and having much finer fibulae. Sovereign (1958) suggested a resemblance with N. gracilis, differing from that commonly reported species by the fine fibulae and their variation in size within a single individual.
BioData Reference
Krammer and Lange-Bertalot 2/2 Pl 66 Fig 1-11