E.Reichardt, Jüttner and E.J.Cox 2004
Gomphonema caperatum and G. insularum are less clavate.
Gomphonema freesei is larger, both in length and breadth.
Gomphonema amerhombicum is less clavate and larger in length and breadth.
Gomphonema christenseni is larger and is astigmate.
Kociolek, Woodward and Graeff 2016
R.L.Lowe and Kociolek 1984
Ponader and Potapova 2017
E.Reichardt 2007