4 WISCONSIN PHYTOPLANKTON 



Apex of semicells not incised. 



Lateral margins deeply incised Micrasterias (p.p.) (p. 40) 



Lateral margins not incised. 

 With spines. 



Cell face with central protuberance Xanthidium (p.p.) (p. 55) 



Cell face without a central protuberance Arthrodesmus (p. 125) 



Without spines. 



Angles continued in processes Staurastrum (p.p.) (p. 62) 



Angles not continued in processes Cosmarium (p. 28) 



Cells radially symmetrical. 



Face with a central protuberance Xanthidium (p.p.) (p. 55) 



Face without a central protuberance Staurastrum (p.p.) (p. 62) 



Cells united to form permanent colonies. 



Colonies filamentous. 



Cell length several times the breadth Pleurotaenium (p.p.) (p. 13) 



Cell length never double the breadth. 

 Cells united by special apical processes. 



Processes interlocking Sphaerozosma (p. 136) 



Processes overlapping adjacent cells Onychonema (p. 135) 



Cells not united by apical processes. 



Without a girdle-like thickening at jioint of division. 



Deeply constricted Spondylosium (p. 139) 



Not deeply constricted Hyalotheca (p. 141) 



With a girdle-like thickening at point of division. 



Length greater than breadth Gymnozyga (p. 146) 



Breadth greater than length Desmidium (p. 143) 



Colonies not filamentous Cosmocladium (p. 37) 



GONATOZYGON De Bary 1856. 



Cells cylindrical, length many times the breadth, without a trace 

 of a median constriction; sides of cells parallel, except near apices 

 where they may be either slightly dilated or convergent ; apices trun- 

 cate, with angles only slightly rounded, at times subcapitate. Cell 

 wall smooth, finely granulate, or clothed with minute spicules or 

 spines. Chloroplasts axial ribbon-like strands extending from pole 

 to pole or from each pole to middle of cell; pyrenoids fairly nu- 

 merous and at regular intervals along the chloroplast. Cell division 

 at any point in the cell wall, the cells frequently remaining attached 

 at apices and forming temporary colonies of variable length. Col- 

 onies readily separating into component cells on being disturbed and 

 always separating prior to conjugation. 



Zygospores spherical and with smooth walls. 



KEY TO THE SPECIES. 



Cell wall granulate (1) G. monotaenium 



Cell wall with spines. 



Spines the same diameter at base and apex (2) G. pilosivni 



Spines broader at bases (3) G. aculeatum 



