WISCONSIN PHYTOPLANKTON 13 



10. Closterium decorum De Brebisson. PI. 53, Fig. 7. 



Mem. d. 1. Soc. imp. d. Sc. nat. de Cherbourg 4: 151, pi. 2, fig. 39. 1856; Kirch- 

 ner, Kryptogamenfl. v. Schles. 2>: 139. 1878; W. & G. S. West, Monogr. Brit. 

 Desm. 1: 184, pi. 17, figs. 7-8; pi. 25, figs. 1-3. 1904. 



Cells fairly large, length 12-15 times the breadth, median portion 

 straight, terminal portion curved ; median area narrowly fusiform 

 and attenuated towards the apices, terminal portions with subparallel 

 sides but fairly stout and with apices truncate. Cell wall yellowish- 

 brown ; without a median girdle but longitudinally striated, the striae 

 extending to cell apices; 15-18 striae visible across the face of the 

 median portion of the cell. Chloroplasts ridged and with an axial 

 row of 7-10 pyrenoids. (Tychoplanktont.) 



Zygospores angular (Kirchner) ; subspherical and smooth-walled 

 (W. & G. S. West). 



Cells 450 IX long ; 27.5 [i broad in median portion, 5 /i broad at 

 apices ; 10 striae in 14 /*. 



Diamond (rrr). 



PLEUROTAENIUM Nageli 1849. 



Cells usually quite large, length several times the breadth, median 

 constriction well defined but not deep, cells always straight; semi- 

 cells never compressed, commonly cylindrical and somewhat attenu- 

 ated towards the poles, very rarely ovate; poles always truncate, 

 usually with angles more or less rounded and, except in rare cases, 

 with a ring of mamillate to conical tubercules; bases of semicells in- 

 flated, at times with lateral margins above the inflations undulate. 

 Cell walls rarely smooth, usually finely to coarsely punctate, never 

 with vertical plications on swollen bases of semicells; walls at times 

 of uneven thickness, the thinner portions producing a somewhat 

 definite pattern. Chloroplasts usually numerous and parietal, in 

 the form of straight to undulate bands that extend the length of the 

 semicell and contain numerous pyrenoids; rarely axial and with 

 longitudinal plates and an axial row of pyrenoids. Parietal chloro- 

 plasts at times fragmenting into small rhomboidal masses each con- 

 taining a single pyrenoid. 



Zygospore formation rare. Zygospores spherical and with smooth 

 walls. 



KEY TO THE SPECIES. ? 



Lateral walls the same thickness. 



Poles without tubercules (1) p. Trabecule 



Poles with tubercules. 



Walls with whorls of tumors (5) P. nodosum 



Walls without whorls of tumors. 



Strongly attenuated at poles (4) P. truncatum 



Not strongly attenuated at poles. 



Sides undulate above basal inflation (3) P. Ehrenbergii 



Sides not undulate above inflation (2) S. subcoronulatum 



Lateral walls with thin areas (6) P. Trochiscwm 



