292 DISCOVERY REPORTS 



and is therefore not so important as was previously supposed. Most of the South 

 Georgia material I examined was correctly identified as belonging to this species, but 

 I now believe that some of the Bellingshausen and Weddell Sea material should have 

 been referred to minute phases of Ch. dichaeta (cf. Hart, 1934, p. 164). 



Chaetoceros sociale Lauder. 



Very typical of the group in its space/time distribution, this species is one of the most 

 important ice-edge invaders of truly oceanic habitats. There, however, it never reaches 

 anything like the extraordinary abundance common in truly neritic areas. It was once 

 observed in almost ' pure culture ' in Deception Island harbour, to the number of about 

 25 million cells per litre, estimated by the drop method. The surface waters were 

 visibly discoloured by it on this occasion. 



Chaetoceros tortissimum Gran. 



Truly neritic and very local. Abundant at the Palmer Archipelago and at Adelaide 

 Island. Rarely along the ice-edge and only where the ice has receded a long way south. 



Fragilaria spp. etc. 



Under this heading I have included those tychopelagic species one normally en- 

 counters only in the immediate vicinity of dispersing pack-ice, among which various 

 species of Fragilaria usually predominate, but many other genera are included— rarely, 

 and always in small numbers. If much of our work had been done in littoral waters it 

 would of course have been necessary to give separate heads for such genera as Lepto- 

 cvlindnis also, but this is unnecessary with the material dealt with here. Most important 

 of the ice forms are : Fragilaria curta Van Heurck, F. linearis Castracane and Fragi- 

 lariopsis siiblinearis (Van Heurck) Heiden and Kolbe. Rarer littoral and ice forms that 

 have been included here when necessary are: Cocconeis, Licmophora, Amphiprora, 

 Amphora spp. etc. Round South Georgia Thalassionema nitzschioides Hustedt, a 

 neritic species characteristic of warmer seas, has also been observed since the earlier 

 work was published, and would require separate treatment if we had more inshore 

 samples to consider. It should also be realized that in the material treated here the 

 larger neritic species of Coscinodiscus and other discoid genera were almost absent. 

 Where important they would also demand separate treatment as constituents of 

 Group III. 



Nitzschia closterium (Ehrenberg) Wm. Smith. 



This is the most ubiquitous and variable of all neritic diatoms. In the Antarctic zone 

 it is commonest far south, in a very minute phase which in fresh samples can often be 

 seen to form chains of from three to twelve frustules. In the ice itself larger solitary 

 phases are usually to be found. We found N. closterium frequently in company with 

 Phaeocystis immediately after the ice melted, though it is apparently almost absent from 

 oceanic waters at other times. Lucas has recently described a similar apparent relation 

 with Phaeocystis in the North Sea (1940, p. 128). It is partly due, no doubt, to clogging 



