Plate P4. Species of the Thalassiosira trifulta group. 1-3. Thalassiosira cf. praeoestrupii Dumont, Baldauf, and Barron (Sample 186-1150B-6R-CC); scanning electron microscope (SEM). (1) Internal view of whole valve showing internal processes of rimoportula (R), marginal fultoportulae (MF), and central fultoportula (CF). (2) Internal process of MF. (3) Internal process of CF with four operculate structures (OP) and four trifultate structures (T). 4-6. Thalassiosira frenguelliopsis Fryxell and Johansen (Sample 186-1151A-54R-CC); SEM. (4) Internal view of whole valve showing internal processes of R, MF, and CF. (5) Internal process of MF. (6) Internal process of CF with four OP structures. 7. Thalassiosira frenguelliopsis Fryxell and Johansen. External view of whole valve showing openings of R and CF (Sample 186-1150B-14R-CC); SEM. 8. Thalassiosira tetraoestrupii Bodén. External view of whole valve showing openings of R and CF (Sample 186-1151A-57R-CC); SEM. 9. Thalassiosira exceptiuncula Shiono. External view of whole valve showing openings of R and CF (Sample 186-1151A-54R-CC); SEM. 10-11. Thalassiosira frenguelliopsis Fryxell and Johansen. (10) Whole valve showing R and CF (Sample 186-1150B-14R-CC); light microscope (LM). (11) Whole valve (Sample 186-1150B-11R-CC); LM. 12. Thalassiosira tetraoestrupii Bodén. Whole valve showing R and CF (Sample 186-1150B-6R-CC); LM.

Plate P4. Species of the Thalassiosira trifulta group. 1-3. Thalassiosira cf. praeoestrupii Dumont, Baldauf, and Barron (Sample 186-1150B-6R-CC); scanning electron microscope (SEM). (1) Internal view of whole valve showing internal processes of rimoportula (R), marginal fultoportulae (MF), and central fultoportula (CF). (2) Internal process of MF. (3) Internal process of CF with four operculate structures (OP) and four trifultate structures (T). 4-6. Thalassiosira frenguelliopsis Fryxell and Johansen (Sample 186-1151A-54R-CC); SEM. (4) Internal view of whole valve showing internal processes of R, MF, and CF. (5) Internal process of MF. (6) Internal process of CF with four OP structures. 7. Thalassiosira frenguelliopsis Fryxell and Johansen. External view of whole valve showing openings of R and CF (Sample 186-1150B-14R-CC); SEM. 8. Thalassiosira tetraoestrupii Bodén. External view of whole valve showing openings of R and CF (Sample 186-1151A-57R-CC); SEM. 9. Thalassiosira exceptiuncula Shiono. External view of whole valve showing openings of R and CF (Sample 186-1151A-54R-CC); SEM. 10-11. Thalassiosira frenguelliopsis Fryxell and Johansen. (10) Whole valve showing R and CF (Sample 186-1150B-14R-CC); light microscope (LM). (11) Whole valve (Sample 186-1150B-11R-CC); LM. 12. Thalassiosira tetraoestrupii Bodén. Whole valve showing R and CF (Sample 186-1150B-6R-CC); LM.