FRESHWATER ALG^B. 13 



figs. 26, 29) the pyreiioids were much Larger and starch-formation was confined to their 

 immediate neighbourhood. It is evident, therefore, that in P. antarcticus, as in some 

 other species of Pleurococcus, the presence of a pyrenoid is a variable feature. A 

 pyrenoid was not observed in the large cells of forma robusta. 



The distribution of the fatty oil in the cells of P. antarcticus is evidently also 

 very diverse. Sometimes it appears equally diffused through the cell-contents, which 

 then present a whitish, highly refractive appearance (figs. 30, 31) ; in other cases it is 

 located between cell-membrane and the contracted contents (figs. 26, 31-33), as already 

 described. A very characteristic form taken by the fat is that shown in fig. 29, 

 where it occurs in the shape of a considerable number of lumps, projecting radially 

 from the periphery of the contracted contents and giving the latter a peculiar spiked 

 appearance ; such cell-groups are very common. Drops of oil are also found between 

 cell membrane and contents in forma robusta (figs. 34, 35). 



The size of the cells is also very variable ; it varies from 7 p. in forma minor 

 (cf. below) to nearly 100 /A in particularly large specimens of forma robusta. The 

 commonest dimension of the cell in my material was about 20-22 p. (in the Wests' 

 material 32 p.}. Not uncommonly all the cells of a group are approximately of the 

 same size (figs. 26, 27, 29, 32), but in other cases the cells of a group are of very 

 unequal dimensions. Occasionally the cells appear arranged in well-marked chains 

 (fig. 28, f. jilamentosa, cf. below). 



In a few cases the contents of the larger cells were tinged a deep reddish- 

 yellow colour. 



On the basis of these observations I give the following emended diagnosis of the 

 species and a list of its principal forms : 



PLEUROCOCCUS ANTARCTICUS W. and G. S. West, emend. 



Cellulis globosis (vel rarius ellipsoideis), in forma typica 10-37 (plerumque 20 

 vel 32) p. latis (sed in aliis formis, 7-100 p. latis), singulis vel in familiis parvis 

 aggregatis, inter algas Myxophycearum libere natantibus vel intra stratum Myxo- 

 phycearum nidulantibus, membrana cellularum usque ad 2'2/i crassa, homogenea 

 vel plus minusve distincte lamellosa ; chromatophora magna, parietali, indistincte 

 limitata, plerumque cum pyrenoide conspicuo ; oleum saepe adest, forma globulorum 

 conspicuorum vel in cytoplasmate diffusum. Propagatio ignota. 



Forma typica (PI. L, fig. 26). 



Cellulis singulis vel in familiis parvis aggregatis, 10-37/x latis, cum pyrenoidibus 

 couspicuis et globulis oleariis inter membranam cellularum et cytoplasma dispositis ; 

 membrana cellularum usque ad 2 2 p. crassa, homogenea vel distincte lamellosa. 



This was the most widely distributed form in the ' Discovery ' material. 



