i64 DISCOVERY REPORTS 



Chaetoceros didymus, Ehrb. 

 Lebour, 1930, p. 103, fig. 97. 

 Forms apparently referable to this species, which has hitherto been regarded as 

 characteristic of warmer seas, were from time to time recorded in all types of Antarctic 

 surface water. Undoubted Ch. didymus was observed at one station near the tropical 

 convergence. 



Chaetoceros flexuosus, Mangin. 



Mangin, 1915, p. 45, fig. 27, pi. i, fig. 7. 

 A rather rare species mainly confined to a few stations to the north-east of the 

 Bellingshausen Sea, reaching its maximum abundance near land. Present also in the 

 eddy of western Weddell Sea water round Joinville Island into Bransfield Strait, but 

 not observed elsewhere in the Weddell Sea. Moderately abundant at one station in the 

 Scotia Sea, in old water of Bellingshausen Sea origin very early in the season. 



Chaetoceros neglectus, Karst. 



Karsten, 1905, p. 119, pi. xvi, fig. 5; Mangin, 1915, p. 47, fig. 29. 

 This minute species was found very abundantly in all types of Antarctic surface water, 

 the only areas in which it was comparatively unimportant being the eastern Weddell 

 Sea, and Bransfield Strait. It was outnumbered by larger species in the former, and 

 mainly confined to the eddy of Weddell Sea water at the eastern end of the latter area. 

 Ch. neglectus was most abundant on the spring survey round South Georgia, being one 

 of the dominant forms in the rich phytoplankton to the south-west, where it was fre- 

 quently present in such numbers as to defy estimation. More common in the Bellings- 

 hausen than in the Weddell Sea farther southward, but frequently abundant in both 



areas. 



Chaetoceros radiculum, Castr. 



Castracane, 1886, p. 79; Karsten, 1905, p. 117, pi. xv, fig. 3; Mangin, 1922, pp. 59-61, fig. 7. 

 Mangin holds that this species should be regarded as a form of Ch. schimperlanus, 

 Karst., as in the chain form only the bristles on the distal valves of the terminal frustules 

 are swollen into the characteristic bulbous projections upon which the species was 

 founded, the individuals in the middle of the chains resembling Ch. schimperlanus very 

 closely. Both the previous workers had noted this fact, however, though Karsten does 

 not refer to the resemblance to the other species. Though we have found chains showing 

 the "intermediate" stage and agreeing with Mangin's figures very closely, it is not at 

 all clear that the species are really one and the same. In the first place, the bristles on 

 the middle cells of the chains, though variable, were always shorter and less strongly 

 curved than those of typical Ch. schimperlanus, and in the second, chains of the latter 

 were frequently present in samples in which prolonged examination failed to reveal any 

 Ch. radiculum. The question is one requiring detailed examination such as is not possible 

 when working through large series of plankton samples. For the present we think it best 

 to retain the two species as distinct. Our intermediates were all found at St. 461 to the 



