206 DISCOVERY REPORTS 



apparatus (NHP) during the circumpolar cruise of the R.R.S. 'Discovery II ' during the summer of 

 1937-8. This has been supplemented by published data from hauls with the Gran international net 

 (N 50 v) obtained in 1929-31 by both the R.R.S. 'Discovery II ' and the R.R.S. 'William Scoresby '; 

 by centrifuge counts, usually from six depths at each station, obtained during the third commission 

 of the R.R.S. ' Discovery II ' (1933-5) '■> an ^ by some of Mr J. W. S. Marr's field notes obtained during 

 the fourth commission (1935-7)- 



Taking the ecological groups in order: 



Group I. Small oceanic pennate diatoms with Distephanus. Here it is seen at once that all three 

 species have a very complete circumpolar distribution. 



Group II. Large diatom species: the solenoids, large Chaetocerids and two elongated oceanic 

 pennate forms, twelve categories considered. Here Chaetoceros criophilum, Rhizosolenia alata, Dactylio- 

 solen antarcticus, both main phase groups of Corethron and Synedra pelagica, all show complete 

 circumpolar distribution, and there is no doubt that a wider choice of stations within the relevant 

 dates between o° and 20 W. would complete the circle for that ' patchy ' species, Thalassiothrix 

 antarctica. The species of Rhizosolenia, other than Rh. alata have a gap here and there, but they would 

 almost certainly show continuous distribution if further observations become available. 



Group III. Neritic/ice-edge forms, seven categories considered. This group necessarily shows less 

 continuous distribution than the others, for the majority of the observations were obtained under 

 strictly oceanic conditions. It has already been shown (Hart, 1942, p. 285) that the importance of this 

 group in the open Southern Ocean is confined to a particularly narrow time-interval immediately 

 following the break up of the pack-ice. It is to this fact and the unavoidable absence of neritic 

 observations in many sectors that the discontinuity of distribution shown here is due. Asteromphalus 

 parvulus shows complete continuity, but this is the most nearly oceanic member of the group. 

 Thalassiosira spp. and Nitzschia closterium, however, show only one gap each, and it is to be supposed 

 that further work would show continuous distribution for the other species of this group also. 



Group IV. Oceanic Chaetocerids of medium size, eight categories. Gaps in the circumpolar dis- 

 tribution of this group as represented here are almost certainly due to their time distribution. Dr Hart 

 has shown (Hart, 1942, p. 293) that they reach their greatest relative importance during the post 

 maximal midsummer decrease of the phytoplankton as a whole — Chaetoceros radiculum even later as 

 already mentioned — and this fact made it impossible to select stations representative of the full spatial 

 distribution of the group. Even so, there is very little doubt of the complete circumpolar distribution 

 of all the categories. Chaetoceros atlanticum and Ch. dichaeta (type phase) show complete continuity 

 from the present data. Ch. castracanei shows one gap, Ch. chunii and Ch. schimperianum only two 

 each. 



From the data presented here the following species show complete circumpolar distribution within 

 the Antarctic zone: Fragilariopsis antarctica*, Nitzschia seriata, Distephanus speculum, Chaetoceros 

 criophilum, Rhizosolenia alata, Dactyliosolen antarcticus, Corethron criophilum, Synedra pelagica*, 

 Asteromphalus parvulus*, Chaetoceros atlanticum, Ch. dichaeta. 



The majority of these are also completely cosmopolitan species, occurring in polar and sub-polar 

 waters of both hemispheres ; most of them in specialized portions of the intervening sub-tropical and 

 tropical waters as well, though not, as a rule, in the same phases. Only the species marked with an 

 asterisk are rigidly confined to the far south, and even they 'overlap' occasionally into the sub- 

 Antarctic zone, especially Fragilariopsis. 



Turning to species of which the circumpolar distribution seems certain though not completely 

 shown by the present data we can add : Rhizosolenia antarctica* , Thalassiothrix antarctica* , Thalassio- 

 sira spp., Nitzschia closterium, Chaetoceros castracanei. 



