In SEM, two types of process, fultoportula and rimoportula, are the useful characters to distinguish both taxa except for the areolae structure as you mentioned.

In LM, though these are not the certain criteria for all thalassiosiroid and Coscinodiscus, it might be useful to distingusih both groups.

(1) diameter: Thalassiosiroids commonly do not exceed 100 µm except for some species such as T. puntigera, T. tumida, T. leptopus etc. While Coscinodiscus frequently exceed 100 µm in a diameter.

(2) central area: Thalassiosiroids commonly have the fultoportula adjacent a sinlge large areola, while Coscinodiscus show the rostte areola or hyaline without any processes.

(3) areola size: Thalassiosiroids finer than Coscinodiscus commonly.

(4) areolation: Thalassiosiroids: linear, fasciculate, eccentric; Cosciondiscus: radial

(5) marginal process: The marginal fultoportulae are discernible in thalassiosiroids, while the marginal rimoportulae indiscernbiel in Coscinodiscus commonly.

Best wishes,

Dear Anton:

A very good book for establishing generic and specific features is Hasle in Syvertsen in Tomas (1997), Identifying Marine Phytoplankton.

I agree with features enlisted by Joon and I listed the main differential features between Thalassiosira & Coscinodiscus based on Hasle & Syvertsen (1997)

 Generic characters of Thalassiosira:

Chains or cells embedded in mucilage.

Cells in chains connected by organic thread(s) extruded from strutted process(es).

Cells usually discoid.

One, two or more rimoportulae, equal among them similar in size.

Marginal ringo of fultoportulae, that look different than rimoportulae seeing with LM.

Valve wall with loculate areolae in various patterns or with faint radial ribs.

 Generic characters of Coscinodiscus:

Cells solitaries

Cells discoids to cilindrical, coin shaped or wedge shaped

Two larger marginal labiate processes (macrorimoportulae).

Marginal ring of smaller labiate processes.

Numerous, usually disc-shaped, chloroplasts.

Valve wall with loculate areolae in radial pattern.

If you have not Tomas (1997) contact to me to [email protected]

Kind regards.

Eugenia

I

 Besides 1-2 rimoportulae, Thalassiosira species have many fultoportulae and several species have occluded processes. The cribra locate close to the internal valve face and the foramens distribute on the external valve face. Contrarily, in terms of Coscinodicus species, the cribra locate close to the external valve face and the foramens distribute on the internal valve face. Without fultoportulae and occluded processes, Coscinodiscus species only have many rimoportulae. Allied Thalassiosira and Coscinodicus taxa should be re-examined and revised to confirm their taxonomic positions. [Quoted from LI Yang. Morphological Characteristics Comparisons Between Thalassiosira and Coscinodiscus[J]. , 2009, 29(3): 282-288. Please check the reference].