zio DISCOVERY REPORTS 



same positions, 1 this arbitrary choice of a level of 'dominance' was found to show the following 

 results : 



(a) The lowest (7th) 'dominant' ranged between 07 and 7-1% of the estimated total, mean 3-5% 

 with aM 1-3. At only two of the seventy-eight stations was its value below 1%, and although the 

 5 % level was exceeded at 8/78 stations, the eighth number of the list (highest category excluded on this 

 scheme) exceeded 5 % of the total once only. 



(b) The sum of the 'dominants', i.e. ^th of the numbers on each list, ranged between 59-8 and 

 99-6% of the estimated totals, mean 81 -6% with aM 10. The level fell below 60% at one station only, 

 below 65 % at 5/78 stations, and it exceeded 95 % at 6/78 stations. 



For the four samples of visibly discoloured surface-water, three examined by the drop method and 

 one with the aid of the centrifuge, the 5% level was chosen as the lower limit of 'dominance'. 



In the primary tables for the first (autumn) survey the stations numbers are given in chronological 

 order as they were worked, from north to south. During the second survey interpolation of extra 

 stations (mostly for bottom-sampling) and the fact that it was worked from south to north destroys 

 the sequence of the serial numbers. The data have, therefore, been listed so that microplankton data 

 from repeat positions are given in the order in which the corresponding stations had been occupied 

 during the first survey. Although this results in the last becoming first, it should not be confusing 

 because, if the serial numbers are disregarded, it also has the result that observations made in equi- 

 valent positions are treated in the same sequence in the Tables for each survey (cf. Figs. 1 and 2, 

 and Tables 14 and 15). 



Stations where full series of hydrological and plankton observations were made have been dis- 

 tinguished by printing their serial numbers in ordinary type, while those where only the micro- 

 plankton net and bathythermograph were used are given in italics. 



As an aid to verbal description, the four main lines of observations worked east and west have been 

 named from the most prominent topographical features near their coastal terminations, in order from 

 north to south : the Mowe Point line, Walvis Bay line, Sylvia Hill line and Orange river line. The three 

 lines of subsidiary observations connecting them have been termed the northern, mid- and southern 

 intermediate lines. Approximate mean latitudes for stations worked on each of the four main lines are : 

 Mowe Point line, 19 40' S.; Walvis Bay line, 22 40' S.; Sylvia Hill line, 25 20' S.; and the Orange 

 river line 28 40' S. 



The order in which groups of organisms, and lesser taxonomic units within the groups, have been 

 arrayed in tables and diagrams, has been governed by the main object of this part of our work: 

 description of the rich microplankton flora as a whole. Strict adherence to the sequence of some 

 recognized scheme of classification would render them very cumbersome and even perhaps misleading, 

 owing to the impracticability of publishing the raw data in full, or of identifying members of all 

 groups down to the same taxonomic level. These difficulties, added to the admittedly unequal sampling 

 of certain groups by the net method, have prompted the following explanation of the relation between 

 our arbitrary arrays and systematic classification, since we have no wish to do violence to system- 

 atists' views, whenever they seem well established. 



First we give the sequence of main groupings represented in the Benguela current samples, all of 

 which, excepting the silicoflagellata, 2 are usually regarded as classes of algae, according to the classifica- 

 tion of Fritsch (1935, 1945) — perhaps the most widely accepted at the present time. 



1 That is omitting the extra stations worked south and north of the main survey area during the second survey. 



2 Fritsch treats the silicoflagellata, whose affinities are still in doubt, as a minor group of uncertain status, at the end of his 

 discussion of the chrysophyceae. We are indebted to Dr M. W. Parke of the M.B.A. Laboratory, Plymouth, for advice on 

 this point. 



