FRESHWATER ALG 2. 13) 
In view of the fact that this characteristic arrangement of the oil-drops was 
noticed in all the cells of certain groups, it appears to be typical of a definite 
5 
form of the species. 
9. PLEUROCOCCUS KOETTLITZI* sp. n. 
(Pl. I., figs. 36-42). 
Cellulis globosis vel saepe plus minusve ellipsoideis, plerumque 11-16 » latis, in 
familiis parvis 1regularibus vel coelastriformibus vel quadricellularibus et tetraedricis 
agoregatis ad superticiem stratuum Phormidii afixis; membrana cellularum tenui, 
sed distincta; contentu homogeneo, obscurato accumulatione magna olei, quod 
semper adesse videtur et facit in familiis cellularum aspectum valde refringentem et 
saepe nigrescentem ; contentu interdum rubescente ; chromatophora singula probabilis 
est, cum pyrenoide. Propagatio divisione contentus cellule in quatuor cellulas filiales, 
quae sunt tetraedrice dispositee. 
Diam. cell. = 11-16 p; cell. ovales plerumque 15 pw long. et 12-13 p lat. 
Hab.—Widely distributed on the Phormidium-sheets, especially from the Gap 
pond, Winter Harbour. 
The most prominent characteristics of this species are the often slightly oval 
shape of the cells, the very prominent storage of fatty oil and the tetrad-like 
grouping of the cells in young and often in older families. Young colonies are not 
uncommonly to be found in which the four cells form an obvious tetrad (figs. 37, 
39, 41); im the older colonies the grouping of the cells is frequently very similar to 
that of such a form as Coelastrum microporum, but, when the colonies consist of very 
large numbers of cells, their grouping becomes irregular (fig. 36). Even in these 
irregular colonies, however, individual groups of cells are often in tetrads. This 
prevalent tetrad-like grouping is undoubtedly due to the method of reproduction, four 
daughter-cells arising within the membrane of the mother-cell (fig. 40, only three 
cells are visible); it appears that some of the cells of the tetrad may occasionally 
abort (cf. fig. 42). As detached pieces of cell-membrane are sometimes found lying 
near the colonies of this species it seems probable that the membrane of the 
mother-cell is ruptured and does not undergo gelatinisation. 
The fatty oil generally obscures the whole of the cell-contents and gives the 
colonies, under the low power of the microscope, a dark highly refractive appearance 
which is very characteristic. Osmic acid gives a deep brownish-black colouration. 
In some of the larger colonies the contents of occasional cells may be tinged 
yellowish-red, without these cells presenting any other point of difference to the 
normal ones. A pyrenoid and a limited number of starch-grains are generally present, 
but the pyrenoid does not appear to be quite constant. The cell-wall is 
generally thin. 
* Named after the collector, Dr. Koettlitz, surgeon to the ‘ Discovery.’ 
VOL, VI. I 
