MICROPLANKTON 257 



offshore. It then became clear that the results from the wide intermediate or 'outer shelf zone 

 (40-100 miles from the land), besides yielding the obvious intermediate averages, also showed up the 

 extent to which surface-waters of the two more distinctive types were overstepping the arbitrary 

 limits chosen on each of the two surveys. The full value of this feature could only be realized by 

 considering individual species on each line of stations as we have already treated the main groups. 

 Though this is impracticable here it still seemed worthwhile to include average values for the arbitrary 

 'outer shelf zone, despite the masking effect of the 'averaging' process in the following tables, for the 

 reason given above. 



For this special purpose the anomalous results from the Orange river line were not considered, 

 and this leaves thirty-two stations at repeat positions on each of the two surveys for comparison. 



Table 22 shows the average estimated diatom totals per net haul, and the average totals and per- 

 centages for each main group as previously defined, when the data are arrayed in this fashion. It also 

 provides in effect a crude summary of the line-upon-line treatment of main group-distribution already 

 given. It is essential that the relative quantities indicated should be kept in mind if the percentages 

 for species within each group, given in Tables 13-17, are to be properly understood. 



Table 22. Average estimated diatom totals, with average numbers and percentages of the main diatom 



groups, when the data are grouped according to distance from the coast as shown. (Results from the Orange 



river line excluded) 



Average 



estimate 



of total Average Average Average Average Average 



diatoms Discineae Biddulphiineae Chaetoceraceae Soleniineae ' Pennatae' 



per net , * , , * , , * , , * * , — * * , 



haul Numbers % Numbers % Numbers % Numbers % Numbers % 



Inshore stations <40 sea miles from the coast 



First survey 85,157,491 2,836,753 3-33 1,528,631 i-8o 68,024,177 79-88 877,582 1-03 11,890,348 13-96 



(autumn) 

 Second survey 65,979,818 884,173 1-34 72,400 o-n 58,486,700 88-64 146,455 0-22 6,390,090 9-68 



(spring) 



'Outer shelf stations 40-100 sea miles from the coast 



First survey 15,712,563 217,934 1-39 14,672 0-09 14,673,800 93-39 80,419 051 7 2 5-73 8 4' 02 



(autumn) 

 Second survey 2,158,972 82,025 3'8° 225 o-oi 811,229 37'57 7 2 7»°9 I 33"6S 538,402 24-94 



(spring) 



Offshore stations > 100 sea miles from the coast 



First survey 156,287 20,972 13-42 None 22,480 14-38 31,510 20-16 81,323 52-03 



(autumn) seen 



Second survey 779,740 17,880 2-29 None 139,260 17-86 515,400 6610 107,200 13-75 



(spring) seen 



From Table 22 it appears that: 



(1) the Discineae were of minor importance offshore during the first (autumn) survey. Elsewhere 

 they formed but a small proportion of the total diatoms, their greater numbers inshore notwith- 

 standing during both surveys. 



(2) The Biddulphiaceae were of very minor importance inshore, mainly in autumn. Very small 

 quantities of this group were observed on the 'outer shelf, and none at the offshore stations. 



(3) The Chaetoceraceae were strongly dominant inshore at both seasons, rather more so in spring 

 than in autumn. They were also strongly dominant on the 'outer shelf in autumn, but much less so 

 during spring. Offshore they were outnumbered by other groups at both seasons. 



(4) The Soleniineae were the dominant group offshore during spring. They ranked second to 



