NOTES ON SPECIES i6i 



Rhizosolenia crassa, Schimp. 



Karsten, 1905, p. 99, pi. xi, fig. 6. 



Rare in this material, being recorded from three stations far to the south in the Weddell 

 Sea in small numbers, and from one station to the north-west of South Georgia. Some 

 of Schimper's notes (Karsten, 1905, p. 68) and recent reports by Mr John indicate that 

 it may occur more abundantly in sub-Antarctic water at certain times of year. 



Rhizosolenia curva, Karst. 



Karsten, 1905, p. 97, pi. xi, fig. 2; R. curvata, Zacharias, 1905, p. 121, phot, in text. 



Karsten's manuscript was received in April 1905, and Zacharias' paper appeared in 

 July, so that it seems that Karsten's name for this species should be given priority. The 

 figure given by Zacharias is very good, illustrating clearly the large size of this species, 

 and the association in which we have found it most abundantly. The only peculiar point 

 in his account is that the sample was said to have been collected some 300 sea miles 

 south of the Horn, i.e. well within the Antarctic Zone, whereas the association so clearly 

 illustrated in the photomicrograph is precisely the same as that which we encountered 

 in sub- Antarctic waters to the south-east of the Falkland Islands. The forms included 

 have, however, been seen rarely to the south of the convergence, so that it is perhaps 

 unwise to stress this point unduly, particularly as to the south-west of the Horn the 

 convergence may lie as far south as 59°. 



In our collections this was a typically sub-Antarctic species, found rarely south of 

 the convergence, usually in an apparently dying condition. Karsten's extracts from 

 Schimper's field notes indicate a similar distribution in the South-eastern Atlantic. 



Rhizosolenia polydactyla, Castr. 



Castracane, 1886, p. 71, pi. xxiv, fig. 2; Mangin, 1915, pp. 73-5, fig. 52. 



This form is distinguished from R. styliformis by the closeness of the rings, the more 

 bluntly rounded apexes of the frustules, their shorter mucron, and their greater diameter 

 in proportion to their length. Often the frustules, united in pairs, are slightly curved as 

 depicted by Mangin. This was found to be the dominant form in the sub-Antarctic 

 association to the south-east of the Falkland Islands in March 1930, and it was observed 

 in the vicinity of the Antarctic convergence on several other occasions. It was also 

 encountered rarely farther south, particularly in the Bellingshausen Sea, whence Mangin 

 had previously reported it, the frustules being in most cases devoid of contents. A reason 

 for this apparently anomalous distribution is suggested in the discussion of the possible 

 effect of sub-surface currents as transporting agents on p. 188. 



Rhizosolenia rhombus, Karst. 

 Karsten, 1905, p. 95, pi. x, fig. 6. 



Observed in moderate numbers at two stations only, in about 60° S latitude, one to 

 the south-east of the South Sandwich Islands, the other in long. 58° W. 



