NO. 10.] FRESH-WATER ALG.E. 57 



Key to Tribes and Genera. 



Tribe I. gonatozygele. Cells elongate, cylindrical, and 

 unconstricted, forming loose filaments. Cell wall with a differ- 

 entiated outer layer, of which the small roughnesses and spines 

 form a part. 



Chloroplasts axile Gonatozygon 



Chloroplasts parietal and spirally twisted .... Genicularia 

 Tribe II. spirot^nie^:. Cells solitary, relatively short, 

 and mostly unconstricted. Cell wall a simple sac, without a dif- 

 ferentiated outer layer. The cell becomes adult by periodical 

 growth. 



i . One chloroplast in each cell 2 



Two chloroplasts in each cell 4 



2. Chloroplast spirally twisted, axile or parietal. .Spirotcenia 

 Chloroplast plane, axile 3 



3. Cells solitary M esotcenium,* 



Cells forming short filaments Ancylonema 



4. Chloroplasts star-shaped, radiating from a central 



pyrenoid Cylindrocystis 



Chloroplasts ridged with longitudinal serrated ridges 



Netrium* 



Description of Genera. 



Gonatozygon D. By. Cells long and straight, not con- 

 stricted, forming filaments which break up at maturity. The 

 cell wall is smooth or covered with minute granules. The 

 two chloroplasts are axile, thin and waving or twisted ; they 

 contain four to sixteen equidistant pyrenoids. The zygospores, 

 which quickly separate from the empty cells, are smooth. 



Spirotasnia Breb. Cells straight, fusiform, ends rounded, 

 not constricted in the middle; cells single or several, covered 

 by a gelatinous envelope. Chloroplasts spiral bands on the 

 inside of the cell wall. 



Mesotaenium Nag. Cylindrical cells, straight or slightly 

 curved, without median constriction. The ends are broadly 

 rounded. The chloroplast is a flat plate, extending from one 

 end of the cell to the other ; occasionally there are two chloro- 

 plasts. 



M. micrococcum (Kiitz.) Roy and Biss., Fig. 219. 



