PEESHWATER ALGJ3. 13 



figs. 26, 29) the pyrenoids were mucli 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 rohusta. 



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 rohusta (figs. 34, 35). 



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

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

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

 material 32 yu.). 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, i. fila7nentosa, 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 AV. and G. S. West, emend. 



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

 vel 32) ju. latis (sed in aliis formis, 7-100 /a 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 diff'usum. Propagatio ignota. 



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



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

 conspicuis et globulis oleariis inter membranam cellularum et cytoplasma dispositis ; 

 membrana cellularum usque ad 2 ■ 2 /x crassa, homogenea vel distincte lamellosa. 



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



