i63 DISCOVERY REPORTS 



Rhizosolenia simplex, Karst. 



Karsten, 1905, p. 95, pi. x, fig. i. 



A rather rare species widely distributed in small numbers in the older Antarctic 

 surface water in the Scotia Sea, and round South Georgia. Very rare farther south, 

 occurring at only three stations in the Bellingshausen and one in the Weddell Sea. 



Rhizosolenia styliformis, Btw. 



Karsten, 1905, p. 96, pi. x, fig. 5. 



Great size variation is exhibited by this species, which appears to be almost cos- 

 mopolitan, occurring in all the types of surface water investigated. The specific identity 

 of the individuals from warm seas with those obtained in the Antarctic is doubtful, but 

 the resemblance in form is so close as to justify their inclusion until further evidence is 

 forthcoming. 



This species formed an important part of the association in water of the eastern 

 Weddell Sea tvpe, in which it was sometimes strongly dominant, being present in 

 enormous numbers. It was also abundant in the older mixed water, mainly of western 

 Weddell Sea origin, to the south of South Georgia during the spring survey. R. styli- 

 formis was very generally present in the Bellingshausen Sea, and in sub-Antarctic waters 

 during March, but in much smaller numbers. A few individuals apparently referable 

 to this species were observed at one sub-tropical and three tropical stations. 



Rhizosolenia torpedo, Karst. 



Karsten, J905, p. 95, pi. x, fig. 2. 



A rather rare species distributed very similarly to R. simplex but absent from the 

 Bellingshausen Sea. 



Rhizosolenia truncata, Karst. 

 Karsten, 1905, p. 97, pi. x, fig. 3. 



The most important member of the genus in the far south, near the fast ice and at 

 the Palmer Archipelago. Moderately abundant at many stations in the Bellingshausen 

 and Weddell Seas. Confined almost entirely to the eddy of Weddell Sea water in 

 Bransfield Strait. Found rarely and in small numbers farther north at the beginning 

 and end of the season. 



Family CHAETOCERACEAE, Schroder, 191 1 



Genus Chaetoceros, Ehrenberg, 1844 



Chaetoceros atlanticus, Cleve. 



Mangin, 1915, pp. 28-34, figs. 9-1 1. 



A very common species of the older Antarctic surface water of comparatively low 

 latitudes, and one of the most frequent invaders of the sub-Antarctic Zone (it is well 

 known from temperate seas in the northern hemisphere). Found also far to the south- 

 ward in both the Weddell and Bellingshausen Seas after mid-season, but in much smaller 

 numbers, being largely succeeded by Ch. dichaeta in higher latitudes. 



