24 6 DISCOVERY REPORTS 



Cells discoid, solitary, small, valves convex, furnished with strong, subpolygona 

 areolation. Areoles large in the centre, decreasing in size as they approach the margin, 

 central area or rosette absent. Chromatophores : few irregular plates. Diameter of 

 valve. 6o-68/x. 



Not a common diatom, more frequent in European waters than in Antarctic. Ob- 

 served occasionally in plankton around South Georgia, neritic. 



Observed at Sts. 475, 477, 479, 507. 



Coscinodiscus eta Karsten. 



Karsten, 1907, p. 366, pi. 37, fig. 3. 



Cells discoid, solitary, slightly convex, large. Valves covered with very fine lines of 

 puncta, central area or rosette absent. Puncta arranged in quincunx, lines radial. 

 Chromatophores: numerous small rounded bodies. Diameter of valve, 130-170/*. 



A tropical diatom usually found in the Indian Ocean ; it has not been recorded pre- 

 viously from South Atlantic waters. It was observed in small numbers only at one 

 station off South Georgia. 



Observed at St. 478. 



Coscinodiscus gigas Ehrenberg. 



Ehrenberg, 1843, p. 412. 



Karsten, 1907, p. 367, pi. 35, fig. 7. 



Hustedt, 1928, p. 456, fig. 254. 



Cells discoid, very large, weakly siliceous. Valves flat, covered with radial areolation, 

 the areoles increasing in size as they approach the periphery. The areolation breaks down 

 in the centre of the valve to form a large irregular stellate hyaline area. Areoles fur- 

 nished with secondary structure, particularly those near the margin of the valve. 

 Chromatophores: numerous small rounded bodies. Diameter of valve 300-500/x, 

 mostly 450/1. 



A large weakly siliceous diatom, oceanic, having a wide distribution in tropical and 

 subtropical seas. Its occurrence in narrow belts of water suggests that it is susceptible 

 to slight changes of salinity. It appears to favour a high salinity. It occurred com- 

 monly, but never in great numbers, all around the coast of South Africa, and again but 

 less frequently off the coast of Brazil. 



Observed at Sts. 260, 261, 262, 721, 722, 723, 1373 ; WS 709, 710. 



Coscinodiscus Grani Gough. (PI. XIII, fig. 2.) 



Gough, 1905, p. 338. 

 Hustedt, 1928, p. 436, fig. 237. 

 Gran, 1905, p. 34, fig. 35. 

 Lebour, 1930, p. 44, fig. 20. 



Cells discoid in valve view, cuneiform in girdle view. Valves covered with fine 

 areolation, having a central rosette. Areoles in radial lines, large at the centre of the 

 valve, decreasing in size as they approach the margin. Margin of valve furnished with 

 a ring of fine spinulae, from which fine hyaline lines proceed to the centre of the valve. 



