Cocconeis pseudothumensis is easy to confuse with C. neodiminuta, because transitional forms may be observed. The valves of C. pseudothumensis are broadly-elliptic with rounded ends and the raphe valve has finer striae that are not resolvable with LM. The rapheless valve has a wider, elliptic-lanceolate axial area and relatively smaller, rounded areolae. Cocconeis disculus is larger, has coarser striae on the rapheless valve, and wider areolae on rapheless valve with different ultrastructure (see Stancheva 2018 for details and images). Cocconeis disculus is not yet confirmed in streams and rivers of California.

LM scalebar = 10 µm
Cocconeis Neodiminuta Lm04 Cocconeis Neodiminuta Lm06 Cocconeis Neodiminuta Lm02 Cocconeis Neodiminuta Lm03 Cocconeis Neodiminuta Lm05 Cocconeis Neodiminuta Lm01
Length Range
7-18 µm
Width Range
5-9 µm
Striae in 10 µm
Raphe valve 24-32, rapheless valve 11-14

Cocconeis pseudothumensis

E.Reichardt 1982

Cocconeis pseudothumensis LM4 Cocconeis pseudothumensis LM3 Cocconeis pseudothumensis LM5 Cocconeis pseudothumensis LM6 Cocconeis pseudothumensis LM1 Cocconeis pseudothumensis LM2
Length Range
9-15.3 µm
Width Range
6.9-10.6 µm
Striae in 10 µm
rapheless: 10-14, raphe: 25-27 at margin, 38-40 near axial area

Also compare to

Cocconeis disculus

BioData Reference
Krammer and Lange-Bertalot 2/4 PL 56 Fig1-13