ANALYTICAL K K\ . 27 



a. The obliquely-fitting new and old parts of the cell-wall at the 

 point of division (the isthmus) remain plane. Solitary or colonial. 

 f After division the cells become free and solitary individuals, 

 t Cells elongated and cylindrical; constriction slight. 

 + Apices of cells truncate or rounded. 

 Bases of semicells plicate. 



Docidium. 

 Bases of semicells plane. 



Pleurotgenium. 



+ + Apices of cells cleft, incision widely open or narrow. 

 Cell-wall adorned with rings of furcate pro- 

 cesses. 



Triploceras. 

 Ceil- wall plane. 



|| Apical incision widely open, apical angles 

 furnished with a spine. 



Ichthyocercus. 



|| || Apical incision narrow, apical angles 

 rounded. 



Tetmemorus. 



tt Cells relatively short, commonly compressed or radia- 

 ting ; constriction usually deep. 



+ Cells compressed (at right angles to the plane of 

 the front view) ; from the vertical view fusi- 

 form or elliptical. 



Cells almost always with an apical incision 

 and a moderately lobed margin ; with a 

 central protuberance. 



Euastrum. 



Cells very compressed, with deeply lobed or 

 incised margins. 



Micrasterias. 



Cells with a more or less entire margin, often 

 furnished with warts or spines. 

 || Cells commonly with a central protuber- 

 ance. 



Ti" Cell- wall either smooth, or granu- 

 late, or verrucose, etc. Central 

 protuberance present or ab- 

 sent. 



Cosmarium. 



^f^[ Cell- wall with regularly arranged 

 spines, generally in pairs. 

 Central protuberance always 

 present. 



Xanthidium. 



|| || Cells without a central protuberance : 

 angles spin ate. 



Art It >-< 



