Thalassiosira visurgis, has only a single central fultoportula and a single ring of marginal fultoportulae. Thalassiosira weissflogii, has 3-6 central fultoportulae and a single, prominent rimoportula on the valve margin. Both T. lacustris and Actinocyclus normanii have an undulate valve face.

LM scalebar = 10 µm
Thalassiosira baltica LM8 Thalassiosira baltica LM6 Thalassiosira baltica LM12 Thalassiosira baltica LM5 Thalassiosira baltica LM3 Thalassiosira baltica LM1 Thalassiosira baltica LM11
18-27 µm

Thalassiosira lacustris

(Grunow) Hasle 1977

Thalassiosira lacustris LM6 Thalassiosira lacustris LM2 Thalassiosira lacustris LM3 Thalassiosira lacustris LM4 Thalassiosira lacustris LM1 Thalassiosira lacustris LM5 Thalassiosira lacustris LM7
13-32 µm

Thalassiosira weissflogii

(Grunow) G.A.Fryxell and Hasle 1977

Thalassiosira weissflogii LM3 Thalassiosira weissflogii LM4 Thalassiosira weissflogii LM1 Thalassiosira weissflogii LM2 Thalassiosira weissflogii LM6 Thalassiosira weissflogii LM5
10-20 µm

Actinocyclus normanii

(Gregory ex Grev.) Hust. 1957

Mac  A Normanii 64  86  Finished Mac  A Normanii 57  68  Finished Mac  A Normanii 46  28  Finished Mac  A Normanii 36  91  Finished Mac  A Normanii 28  67  Finished 416 3 Gv Crop
14-72 µm

Also compare to

Thalassiosira visurgis

Hust. 1957