NOTES ON SPECIES 293 



of the filtering apparatus by the Phaeocystis jelly, which increases the chances of the 

 minute Nitzschia closterium being retained. Our centrifuge samples, however, showed 

 that although present elsewhere when not captured in nets, TV. closterium was definitely 

 abundant in the same areas as Phaeocystis. This cannot be ascribed to more complete 

 sedimentation in the centrifuge tubes due to presence of Phaeocystis, because the 

 plankton was rich enough to enable us to work with volumes of water so small that 

 Phaeocystis colonies were quite often not included. It seems likely, therefore, that the 

 association is a real one, as Lucas is inclined to believe. Such quantities of Nitzschia 

 closterium as have been captured by our net methods, which admittedly are not adequate 

 for such a small frequently solitary species, shows a time distribution typical of our 

 neritic/ice-edge grouping. 



Group IV 

 Chaetoceros atlanticum Cleve. 



The most important member of the group in the northern region of the Antarctic 

 zone, this cosmopolitan oceanic species shows its greatest absolute abundance in areas 

 subject to neritic influence at the time of the main increase. Its importance relative to 

 the other phytoplankton present, however, is typical of the group, being greatest during 

 the post-maximal decrease and in autumn, in oceanic regions. Ch. atlanticum diminishes 

 in importance as one proceeds southwards, but even in the southern region small 

 numbers are to be found from time to time. 



Chaetoceros castracanei Karsten. 



To be found in all parts of the Antarctic zone, and its time of maximum relative 

 importance is the same as that of the other oceanic chaetocerids— post-maximal, not 

 earlier as with all the members of the neritic/ice-edge group. Ch. castracanei increases 

 in importance as one proceeds southwards. 



Chaetoceros chunii Karsten. 



The time distribution of this species shows it to be most important during the post- 

 maximal period in all parts of the Antarctic zone, i.e. long after the ice has receded in 

 the oceanic regions. No doubt its absolute abundance may be greater in neritic areas 

 earlier in the year, but almost all Antarctic plankton diatoms reach their greatest 

 abundance in neritic areas at the time of the main increase, and I am sure no one would 

 proceed to describe them all as neritic species for that reason alone. Ch. chunii is widely 

 distributed, rather more important in the northern regions and areas than farther south. 



Chaetoceros curvatum Castracane. 



This oceanic, usually solitary species, seems to find its optimum in sub-Antarctic 

 and perhaps sub-tropical waters. It was found, however, in small numbers throughout 

 the year in the Northern and Intermediate Regions of the Antarctic zone. Very rare 

 farther south. 



