52 



DISCOVERY REPORTS 



be that the meshes of the N 70 V nets on the November line were considerably choked 

 by the large number of Thalassiosira present in the catches. 



It was present in smaller numbers between South Georgia and the Falkland Islands, 

 and absent from St. 70 across the Antarctic Convergence; this is shown in Fig. 20. It 

 was only present in comparatively small numbers on the autumn C line taken in late 

 May— see Appendix I, Sts. WS 110-1 14. Mangin (1915) has shown that it dies down at 

 the end of April and is very rare from May to October, its period of abundance being 

 from December to April. In Appendix I its complete distribution as shown by N 50 V 

 nets is given. 



Karsten (1905) has distinguished from the typical Corethron valdiviae a more slender 

 species under the name C. inerme. Mangin writes that the characters he (Karsten) has 

 given it are so difficult to apply in the 

 presence of slender forms of C. valdiviae 

 that, except in a few particular instances, 

 it is impossible to distinguish them. In 

 our present survey all the forms appear 

 to belong to C. valdiviae; but in the 

 methods of analysis dealing with such 

 enormous numbers it has been impossible 

 to be certain that C. inerme has not been 

 included amongst some of the slender 

 forms of C. valdiviae. 



Rhizosolenia. 



Twelve species have been recorded. 



Rh. alata, Brightw., has occurred on 

 both sides of South Georgia; its distri- 

 bution is shown in Appendix I. Since 

 considerable confusion has arisen in the 

 literature over this diatom and Rh. obtusa, 

 Hensen, we have figured both from draw- 

 ings made from specimens in the survey 

 so that there can be no doubt as to the 

 forms we mean by these names. Rh. alata 

 is shown in Fig. 21. They were usually 

 30—35 /x in diameter, but more slender 

 forms of I2ju. have not infrequently been 

 taken and in a few instances forms as 

 slender as 8 /a. 



Rh. antarctica, Karsten, was only re- 

 corded at three stations on the western 

 side of South Georgia: 161, WS 43 and 

 WS44. 



^c 



-LJ 



60 1 



lllllll 



60 L 



Fig. 



Rhizosolenia alata, Brightw., drawn from 

 specimens in the survey. 



