FRESHWATER ALG^l. 15 



In view of the fact that this characteristic arrangement of the oil-drops 

 noticed in all the cells of certain groups, it appears to l>e typical of a definite 

 form of the species. 



9. PLEUROCOCCUS KOETTLITZI* sp. n. 

 (PI. I., figs. 3G-42). 



(Vllnlis idobosis vel saepe plus minusve ellipsoideis, plcrumque 11-16 p. latis, in 

 familiis parvis irregularibus vel coelastriformibus vel quadricellularibus ct tetraedricis 

 aggregatis ad superticiem stratuum PlwnniiHi afiixis ; membrana cellularum tenui, 

 scd distiucta ; couteutu homogeueo, obscurato accumulatione magna olei, quod 

 semper adesse videtur et facit in familiis cellularum aspectum valde refriugeutem et 

 saepe nigrescentem ; contentu interdum rubescente ; ehromatophora singula probabilis 

 est, cum pyrenoidc. Propagatio divisioue couteutus cellular in quatuor cellulaa tiliales, 

 quae suut tetraedrice dispositas. 



Diam. cell. = 11-1 G /A ; cell, ovales plerumque 15 /A long, ct 12-13 p. lat. 



J/iif>. Widely distributed on the Phormidium-aheebB, 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); in the older colonies the grouping of the cells is frequently very similar in 

 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 arc visible) ; it appears that some of the cells of the tetrad may occasionally 

 abort (cf. fig. 42). As detached pieces of cell-membrane arc 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 gelatinisatiou. 



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 arc generally present, 

 but the pyrenoid docs not appear to be quite constant. The cell-wall is 

 generally thin. 



* Named after the collector, Dr. K.H ttlitz, surgeon to the 'Discovery.' 



VOL. VI. I 



