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Nitzschia tropica is more linear and has a straight raphe, as compared to N. fonticola. Nitzschia tropica has not verified from collections in the US.

LM scalebar = 10 µm
Fonticola 001 Fonticola 004 Fonticola 002 Fonticola 011 Fonticola 005 Fonticola 003 Fonticola 007
Length Range
10-55 µm
Width Range
2.5-4.5 µm
Striae in 10 µm
24-27

Also compare to

Nitzschia tropica

Hust. 1949