MICROPLANKTON 259 



The Biddulphiaceae (Table 24) were very sparsely represented in the area surveyed. Inshore, in 

 autumn, Eucampia zoodiaais was of some slight importance, and just extended on to the outer shelf 

 area. There, however, Cerataulina pelagica formed a slightly higher proportion of the very small group 

 average. Eucampia zoodiaais was not observed during the spring survey, when Biddulphia longicruris 

 provided the bulk group totals. Biddulphiaceae were not observed more than 100 miles offshore during 

 either survey, though one of the species recorded, Hemiaidus hauckii, has been seen far out in the 

 South Atlantic on other occasions, and should probably be regarded as an oceanic form. 



Table 24. Relative importance of the group Biddulphiaceae, and percentage of the several species within 

 the group. Results arrayed according to distance from the land as shown, omitting those from the Orange 

 river line 



(a) Average total diatoms 



(b) Average total Biddulphiaceae 

 bja as percentage 



Average for each category as % of (b) 

 Biddulphia longicruris 

 Cerataulina pelagica 

 Triceratium favus 

 Hemiaidus hauckii 

 Eucampia cornuta 

 E. zoodiacus 



The tremendous local importance of the Chaetoceraceae is clear from Table 25, which also illu- 

 strates the very cosmopolitan nature of the species of this family. All of those which we were able to 

 identify are widely known from other regions, ranging from ' boreal ' to ' warm temperate ', with some 

 which extend into truly tropical surface-waters as well. 



Within this area most of them were essentially neritic — inshore and outer shelf species — and of 

 these a majority were more abundant in autumn, or late in the succession, than in spring. Most 

 important of all, however, were three panthalassic species which must be among the most widespread 

 and abundant of all marine plankton diatoms : Chaetoceros compression, C. constrictum and C. curvi- 

 setum. The first two of these were almost equally abundant at both seasons, while C. curvisetum, the 

 most numerous of all the diatoms according to these estimates, showed a distinct maximum in spring, 

 though vast numbers had been found in the autumn samples also. 



Of the more strictly neritic species C. didymum was the most abundant, especially in autumn, when 

 vast numbers of the very characteristic resting spores were present. C. subsecundum and most of the 

 other neritic species of lesser but still considerable local importance, for example, C. affine, C. costatum, 

 C. debile, C. teres, C. tetras, showed a similar time distribution. C. strictum, however, showed a strong 

 inshore maximum during the spring survey. 



The most abundant and widespread of the oceanic species, C. convolutum, showed a spring maxi- 

 mum here. So also did two oceanic forms of lesser importance in these samples : C. atlanticum and 

 C. peruvianum. 



The ' ecological characterization ' of the species of this most important and difficult genus of marine 

 plankton diatoms arrived at by the late Professor Gran, through continued efforts towards improving 

 his system of ' plankton elements ' as applied in the northern hemisphere (Gran in Murray and Hjort, 

 1912; Gran and Braarud, 1935), can thus far be extended to this southern area without serious 



18 DHJC 



