12 F. E. FRITSCH. 



PLEUROCOCCACE.E. 



Genus PLEUROCOCCUS Menegli. 

 7. PLEUROCOCCUS FRICJIDUS. 



(PI. I., figs. 43, 44.) 

 Pleitrococcus frlgidm W. and G. S. West, op. cit., p. 270, PL XXIV., figs. 40-44. 



Diam. cell. = 20-22 /*. 



IJab. Freshwater pond among eskers four miles north of Black Island, upon ice, 

 McMurdo Strait, September 12th, 1902; Gap pond, Winter Harbour, February 20th, 

 1904. 



This form was rare and was always found attached to the Phormidtum- sheets. 



8. PLEUROCOCCUS ANTARCTICUS. 



(PI. L, figs. 26-35.) 

 Plnirococcus untarcticm W. and G. S. West, op. cit., pp. 27G-277, PL XXIV., figs. 49-51. 



Ilab. Very widely distributed on the P/tonnidium-sheets, especially Gap pond, 

 Winter Harbour, ponds in eskers near Black and Brown Islands, etc. 



This is a very striking and characteristic constituent of the epiphytic vegetation 

 of the abundant Phormidium-sheets, but obviously much more variable than Messrs. W. 

 and G. S. West's material would lead one to suppose. According to their diagnosis the 

 characteristic features of the species are : the large spherical cells, which are isolated or 

 aggregated in small families ; the thick cell-wall, which is homogeneous or obscurely 

 stratified ; a large, indistinctly limited, parietal chloroplast ; the frequent presence of 

 drops of oil. The form, they observed, was free-floating, but they also distinguish a 

 forma robusta (op. cit., p. 276, PI. XXIV., figs. 52-54), which grows on the surface of 

 the Cyanophyceous strata ; this form has huge cells with a very thick, stratified 

 membrane and is stated to lack drops of oil. 



I have observed no free-floating specimens of .P. antarcticus at all, probably 

 because my Antarctic material was mainly such as had grown attached to some 

 substratum. There occurred, however, on some of the Phormidium-sheets groups of 

 cells agreeing in all respects (except in the attached habit) with Messrs. West's 

 description (f. simplex, cf. below). These cells varied in diameter between 18 and 35 /u, 

 the commonest dimension being 20 jj. ; they contained no pyrenoid, but often numerous 

 starch-grains ; and globules of oil varying in size and number were often present 

 between the thick walls and the contracted contents (PI. I., figs. 30, 31). 



In other groups of cells, however, one (rarely two) pyreuoids were conspicuous 

 after staining with iodine, these cells otherwise agreeing in all respects with those just 

 described (PI. I., fig. 32, f. f'/picn, </. below). In other groups of cells again (PI. L, 



