294 DISCOVERY REPORTS 



Chaetoceros dichaeta Ehrenberg. 



An oceanic, cosmopolitan species showing great variation in size and form. One of 

 the most important members of the group, especially in autumn, in all parts of the 

 Antarctic zone. It is much more common in the extreme south than Ch. atlanticum 

 and tends to alternate with that species in its space/time distribution elsewhere. 



Chaetoceros dichaeta tenuicornis phase. 



I use this term to describe the minute form of Ch. dichaeta which is perhaps the most 

 numerous oceanic chaetocerid of the Antarctic zone. The characteristic flexure of the 

 bristles that led Mangin (19 15, p. 43) to describe it as Ch. dichaeta forma tenuicornis is 

 a variable character, however, and is not shown by all individuals. The phase usually 

 occurs in short chains of three to six frustules, but longer ones are quite common. It 

 has certainly been confused with Ch. neglectim in some previous work, including my 

 own (Hart, 1934) (see note on the latter species in this paper). Ch. dichaeta tenuicornis 

 phase shows a marked increase in relative importance as one proceeds southwards, 

 and is the most important member of the group in the southern region. It is abundant 

 from the time of the main increase onwards, with maximum relative importance much 

 later than the Group III forms. 

 Chaetoceros pendulum Karsten. 



Widely distributed in the Antarctic zone but in very small numbers relative to the 

 rest of the phytoplankton present. I have here treated it as oceanic rather than neritic 

 as Hendey has done, but it reaches its maximum relative importance earlier than other 

 Chaetocerids so that his opinion may be the sounder. If so it should be transferred to 

 Group III, but it occurred in such small proportions in our catches that such a change 

 would not affect the general picture presented. 

 Chaetoceros radiculum Castracane. 



An oceanic species found in all parts of the Antarctic zone in relatively small numbers. 

 The bulbous swollen bristles of the solitary cells, and of the terminal cells of the short 

 chains, are sometimes recognizable in bottom deposits. A peculiar phase, at first 

 suspected of being a new species, was sometimes seen far south. The cells were broad, 

 very weakly silicified, having a very hyaline appearance and strongly accentuated 

 octagonal outline in girdle view; the bristles short and degenerate, often almost invisible. 

 This phase was only seen in rather long chains which evidently broke up easily, but at 

 length some were found with the swollen terminal bristles so characteristic of the 

 species. Ch. radicubmi is never a major constituent of the phytoplankton as a whole, 

 but reaches its greatest relative importance in autumn in the Northern and Inter- 

 mediate Regions. 

 Chaetoceros schimperianum Karsten. 



Hendey is possibly right in regarding this species as neritic rather than oceanic— its 

 time distribution in the open oceans is nearer to that of Group III than that of the 

 majority of our Group IV species, but it was so widely distributed that we have 

 regarded it as oceanic. It decreases in relative importance as one proceeds southwards. 



