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Abstract
The planktonie diatom species Chaetoceros didymus Ehr. and C. protuberans Lauder have been studied by light, scanning and transmission electron microscopy, from samples from the Gulf of California and coasts of Baja California. The two are closely related and some others have combined the entities as C. didymus, but from recent evidence and the present observations it appears that both species should be considered as valid. The main differences are: the life cycle, the morphology of the resting spores, the possesion of capilli on the bases of the setae either in terminal and intercalary valves and a ring of minute spines surrounding the rimoportula at terminal valves (C. didymus), and the presence of a process in the margin of some intercalary valves (C. protuberans). It is, however, recommended that a revision of the type material is necessary to finally clarify the taxonomy. An emended diagnosis of the section Protuberantia is also given.