Fragilariforma virescens is larger, wider, has rostrate ends and has marginal spines. Fragilariforma bicapitata has less capitate ends and a lower stria density.

The iconotype drawing of F. virescens var. capitata (Østrup 1910, Pl. 5, Fig. 125) is smaller, more narrow and has more rostrate-capitate ends and nearly parallel valve margins.

LM scalebar = 10 µm
Fragilariforma marylandica LM1 Fragilariforma marylandica LM2 Fragilariforma marylandica LM3 Fragilariforma marylandica LM4 Fragilariforma marylandica LM5 Fragilariforma marylandica LM6 Fragilariforma marylandica LM7
Length Range
16.4-25.7 µm
Width Range
4.7-6.1 µm
Striae in 10 µm

Fragilariforma virescens

(Ralfs) D.M.Williams and Round 1988

Fragilariforma virescens LM6 Fragilariforma virescens LM3 Fragilariforma virescens LM1 Fragilariforma virescens LM4 Fragilariforma virescens LM2 Fragilariforma virescens LM5
Length Range
15-60 µm
Width Range
6.5-7.0 µm
Striae in 10 µm

Fragilariforma bicapitata

(A.Mayer) D.M.Williams and Round 1988

Fragilariforma bicapitata LM2 Fragilariforma bicapitata LM3 Fragilariforma bicapitata LM4 Fragilariforma bicapitata LM1 Fragilariforma bicapitata LM5 Fragilariforma bicapitata LM6 Fragilariforma bicapitata LM7
Length Range
9.1-51.1 µm
Width Range
2.9-5.5 µm
Striae in 10 µm

Also compare to

Fragilaria virescens var. capitata

Østrup 1910