290 DISCOVERY REPORTS 



by production of microspores, for the proportion of large individuals in these far 

 southern populations is never very high. The large individuals would be described as 

 hystrix phase, with slighter cell walls than usual, in Hendey's terminology. 



A thorough biometric survey of our abundant material of this species would be 

 extremely interesting, but would be far too big a study in itself for inclusion in work 

 upon the phytoplankton as a whole. 



Synedra pelagica Hendey (1937, p. 335) = '5- spathulata Schimper; Karsten, 1905; 

 Hardy (Hardy and Gunther, 1935); Hart, 1934; non S. spathulata O'Meara. 

 Never so abundant as Thalassiothrix antarctica, it is of very similar habit but more 

 usually solitary, rarely forming rafts. It is more widely distributed and more definitely 

 oceanic than that species, with which it is easily confused. In general its range is more 

 southerly and it is not found in dense local concentrations. 



Thalassiothrix antarctica Karsten (Hendey, 1937, p. 335)= TA. antarctica Schimper; 

 Karsten, 1905; Hardy (Hardy and Gunther, 1935); Hart, 1934. 

 The larger individuals of this robust oceanic species are among the longest diatoms 

 known — up to 5 mm. It is particularly abundant at the time of the main increase in the 

 rich mixed plankton of the South Georgia area, but is also to be found throughout the 

 whole of the Antarctic zone. It is commoner in the Northern and Intermediate Regions 

 than farther south and fills even large-meshed plankton nets when abundant. It is 

 frequently colonial, the cells being joined by their truncated ends in rafts, usually in 

 multiples of two up to twenty-four individuals; ' eights' are the most common. Strongly 

 silicified, but the recognizable remains in bottom deposits are mostly fragmentary. 

 Uniformly small and less robust individuals, mostly solitary, have been seen when 

 changes in the Corethron population also suggested shortage of silica. Possibly confused 

 with Thalassiothrix longissima Cleve and Grunow, at some stations near the northern 



limit of its range. 



Group III 

 Thalassiosira spp. 



Most of the Antarctic members of this genus may be referred to Thalassiosira 

 antarctica Comber and Th. subtilis (Ostenfeld) Gran, but Th. gravida Cleve also has been 

 recorded from the South Georgia area by Hendey. Th. antarctica is very variable and 

 certainly at times confused with the much rarer neritic species Coscinosira antarctica 

 Mangin. For descriptions and synonymy of the species of Thalassiosira the reader is 

 referred to Hendey (1937, pp. 237-40). In general the genus is strongly neritic but 

 occurs in smaller quantities in the open oceans immediately after the break-up of 

 the pack-ice. The time distribution is very well marked, occurrence of the genus in 

 any quantity being rigidly confined to the early part of the main increase up to the 

 maximum. A majority of the northern members of the genus appear to have a similar 

 time distribution, being referred to by several writers as markedly spring forms. In 

 the far south Thalassiosira is most important round South Georgia and in other neritic 

 areas. 



