Abstract: Thalassiosira(Cleve) Hasle and Coscinodiscus(Ehrenberg) Hasle & Sims are large and typical genera in diatoms. Due to many similar morphological features, it is easy to misidentify between these two genera. Under light microscope and electronic microscope, their morphological characteristics comparisons have been studied. 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.

Key words: Thalassiosira, Coscinodiscus, fultoportule, rimoportule, cribrium, foramen