i8 DISCOVERY REPORTS 



the Antarctic. From the samples here examined it would appear that the majority of 

 these are littoral and ice forms. Mangin (1915, pp. 51-2) has already remarked on the 

 great difficulty of identifying these forms satisfactorily when working on a large series 

 of plankton samples. Special mounting methods would be necessary to obtain the 

 requisite definition of the fine striations and punctae of the valves, besides which it is 

 often necessary to examine them in girdle view. This would be manifestly impossible 

 when estimating the numbers in a large series of plankton hauls. It is evident that the 

 systematics of the southern members of the genus are badly in need of overhaul, but 

 this in itself would be a labour of years. Bearing in mind the fact that except at a few 

 inshore stations they never form an important part of the phytoplankton, it has been 

 considered justifiable to lump them together as Coscinodisciis spp. with the important 

 exception of C. bouvet. 



The smaller dinoflagellates at no time formed any appreciable proportion of the 

 phytoplankton collected within the Antarctic Zone (cp. Mangin, 191 5, p. 86, and Hardy, 

 in press) and were not identified specifically. A few larger ones were sometimes im- 

 portant towards the northern limit of the Antarctic surface water, but their identification 

 presents no difficulties. 



One or two systematic points should perhaps be mentioned here. The first refers to a 

 form which is perhaps the most numerous and widely distributed of all Antarctic 

 phytoplankton organisms — Corethron valdiviae, Karsten. Both Mangin (1915, p. 51) 

 and Hardy found it impossible to distinguish this form from the C. inerme of the 

 same author, and in the course of the present work this difficulty has again been 

 encountered, the range of individual variation, especially in size, being great. It is 

 probable that this last character is due simply to alternate propagation by means of 

 microspores and auxospores. For this paper the practice has been followed of referring 

 to all single frustules with the double crown of spines as C. valdiviae. Two other 

 forms of C. valdiviae were noted, those in process of forming auxospores, and a thin 

 spineless chain form, which appears to be a vegetative phase derived from the smaller 

 spiny individuals, as intermediates have been observed. These two forms have been 

 estimated separately. In all the analyses, the estimated numbers refer to the individual 

 frustules, not, in the case of colonial and chain forms, to the aggregates of individuals. 



Mangin (1915, p- 62) has shown conclusively that Karsten's two species, Eiicampia 

 balaiistiimi and Moelleria a?itarctica, should be referred to as types of one species, 

 Eiicampia antarctica, as he found both occurring in the same chains. This was also very 

 apparent in our catches: it was frequently impossible to refer to the separate types, as 

 so many intermediates occurred. In general, however, the type moelleria was the most 

 abundant in all samples taken during the summer months, in agreement with Mangin 's 

 findings. The further subdivision of this species into two varieties, according as to 

 whether the chains are straight or spirally curved, appears to be of doubtful value. 



The tables of analyses in this paper have been constructed so as to show the station 

 number, position and date, together with the (ship's) time when a series of hauls was 

 made at one station. On the close lines worked round South Georgia, the positions have 



