NOTES ON SPECIES 287 



paper. Where it is abundant, the chains of Nitzschia seriata are often very long, but 

 break up in preserved samples. A very cosmopolitan species. 



Distephanus speculum (Ehrenberg) Haeckel. 



This widely distributed silicofiagellate is very common in the Antarctic zone, whereas 

 Dictyocha is scarcely ever found south of the convergence. Distephanus was abundant 

 at the same times and places as Group I diatoms with perhaps a stronger tendency to 

 increase in relative importance near the ice-edge. Great variation in form and in size 

 were to be seen where it was abundant. It has been found in pack-ice, but it is not 

 certain that the individuals were ahve. 



Group II 

 Chaetoceros boreale Baily. 



Comparatively rare in this material but sometimes occurred in considerable quantity 

 along with Ch. criophilum, with which it may sometimes have been confused in counting 

 the contorted chains in rich mixed samples — oceanic. 



Chaetoceros criophilum Castracane. 



This oceanic species often occurs in dense local concentrations, sometimes in 

 company with other large forms such as Corethron criophilum. It tends to increase in 

 importance as one proceeds southwards. The long strong bristles contain chloro- 

 plastids and are triturated and swallowed by some of the common Calanoids and 

 Euphausians in spite of their formidable spinose armature. There have been occasions 

 late in the season when observations suggested that this species was dying off. The 

 endochrome turned brown and appeared to degenerate, and the water was full of 

 broken spine fragments, apparently sinking. A chemical analysis of some material 

 dried at about 120° F., carried out by Mr W. J. Copenhagen, showed that the fragments 

 contained an extremely small amount of organic matter. Since it is certain that the 

 spines, which may be up to a millimetre in length, must be bitten off before plankton 

 animals can swallow this species, it may be that rapid break-up of faeces after heavy 

 grazing, rather than death from senescence, was responsible for this state of affairs. 



Rhizosolenia spp. (see Hendey, 1937, pp. 309-20 for synonymy). 



These are all essentially oceanic forms within the Antarctic zone. Rh. hebetata Baily, 

 semispina phase, and Rh. data Brightwell, usually in the gracillima phase, are important 

 in local concentrations, mainly in the Intermediate and Southern Regions. In early 

 work stouter individuals of the first named were confused with Rh. styliformis Bright- 

 well. Among the smaller forms, Rh. antarctica Karsten (not treated by Hendey) and 

 Rh. chunii Karsten have been seen in extremely long chains when fresh material was 

 examined— up to twenty-eight and forty-one frustules respectively. Some of the larger 

 and rarer species seem very characteristic of the older and warmer Antarctic surface 

 waters. Rh. hidens Karsten and Rh. simplex Karsten, in particular, seem confined to the 

 Northern Region and northern half of the Intermediate Region. Except for the local 



