i 5 6 DISCOVERY REPORTS 



Many samples of the plankton from the tropical and subtropical Zones contained 

 large numbers of small crustaceans together with many species of dinoflagellates, 

 tunicates and ciliates. 



The chief representatives of the dinoflagellates were Orniihocercus spp., Ceratium 

 tripos and varieties, C.fusus, C. azoricum, C. bucephalum, Goniaulax spp. and Peridinium 



spp. 



The ciliates belonged to the suborder Tintinnoinea, and the following genera were 

 well represented : Epiplocylis, Tintinnus, Parundella, Rhabdonella, Rhabdomllopsis and 

 Xystonella. 



It is difficult to assess the value of the analysis of a plankton sample, particularly if 

 obtained by means of a vertical haul, and it can in no way be regarded as representing 

 the true population of any given area. The presence or absence of any particular species 

 in a sample must be regarded as fortuitous to a high degree. This is particularly true of 

 samples taken at tropical or subtropical stations, where the total volume of the phyto- 

 plankton is small and the number of diatoms per sample very few, while the number of 

 species present is usually relatively high. In polar areas where the diatom population is 

 in great preponderance, and species relatively few, the analyses of samples taken from 

 a number of stations in close proximity would produce fairly constant results. 



Due consideration must be given to these points when analytical results of different 

 stations within the same zone are compared. 



The dominance of the dinoflagellates, ciliates and crustaceans was well maintained 

 to as far south as 44 S, where a marked increase in the proportion of the diatoms was 

 noticed. St. 666 in latitude 49 58!' S showed a great variety of diatoms and a decrease 

 in the number of individuals and species of the flagellate-ciliate association, but the 

 crustacean population suffered little loss of representatives. At St. 663, however, 

 crustaceans were encountered but rarely, and an entirely different diatom population 

 became dominant. The volume of the samples at all stations south of St. 663 showed 

 great increase, and the characteristic Corethron-Chaetoceros associations frequently 

 amounted to over 90 per cent of the total phytoplankton. 



When this work was first undertaken it became quite clear that an examination of all 

 the available Atlantic material was out of the question. But the utilization of certain 

 hydrological data made it possible to divide the ocean into geographical areas which may 

 be dealt with independently. In order to appreciate the general plan of the work it is 

 necessary briefly to consider the data. 



According to Deacon's paper (1933) the surface waters of the South Atlantic Ocean 

 may be divided into four distinct geographical zones which differ from one another in 

 temperature, salinity, and other factors. These zones are comparatively sharply defined 

 and marked differences in temperature and salinity are recorded when crossing a con- 

 vergence from one zone to another. 



The Antarctic Zone extends from the ice shelf to the Antarctic convergence, which 

 reaches as far north as approximately 47 40' S in the meridian of Greenwich, falling 

 slightly southwards towards 30 E. To the west it passes between the Falkland Islands 



