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Gomphonema apuncto lacks a stigma.

Gomphonema freesei has coarse striae.

Gomphonema amerhombicum is more clavate and larger in size.

Gomphoneis rhombicum has a stria density of 9.5-12 in 10 µm, length of 35-53 µm and width of 4.5-7.5 µm. Note that G. rhombicum has not been verified in North America.

LM scalebar = 10 µm
Gomphonema stoermeri LM7 Gomphonema stoermeri LM2 Gomphonema stoermeri LM3 Gomphonema stoermeri LM5 Gomphonema stoermeri LM1
Length Range
19-45 µm
Width Range
4-6 µm
Striae in 10 µm
13-15

Gomphonema apuncto

J.H.Wallace 1960

Gomphonema apuncto LM4 Gomphonema apuncto LM1 Gomphonema apuncto LM3 Gomphonema apuncto LM2 Gomphonema apuncto LM5
Length Range
21.0-30.7 µm
Width Range
3.0-4.1 µm
Striae in 10 µm
13-16

Gomphonema freesei

R.L.Lowe and Kociolek 1984

Gc53921  Gomphonema Freesei 1 Gc53921  Gomphonema Freesei 2 Gc53921  Gomphonema Freesei 3 Gc53921  Gomphonema Freesei 4 Holotype Gc53921  Gomphonema Freesei 4B Holotype Gc53921  Gomphonema Freesei 5
Length Range
35.0-53.9 µm
Width Range
6.5-7.5 µm
Striae in 10 µm
12.1-14.5 at valve center, 14.2-16.2 at headpole, 11.7-15.4 at footpole

Gomphonema amerhombicum

E.Reichardt 2007

G Amerhombicum  B Isotype 20170301 06 Cl G Amerhombicum  A Isotype 20170301 08 Cl G Amerhombicum  A Isotype 20170301 18 Cl G Amerhombicum  B Isotype 20170301 05 Cl G Amerhombicum  B Isotype 20170301 14 Cl G Amerhombicum  B Isotype 20170301 09 Cl G Amerhombicum  B Isotype 20170301 21 Cl
Length Range
22.5-60 µm
Width Range
5.6-7.7 µm
Striae in 10 µm
13-15 at mid-valve, up to 17 at headpole

Also compare to

Gomphoneis rhombicum

(Fricke) Merino, García, Hern.-Mar. and Fernández 1994