HOW TO KNOW THE FRESH-WATER ALGAE 



450a (345) Shells mostly circular or polygonal in outline (sometimes 



irregular), but with decorations (rows of dots, lines, etc.) which 



are concentrically arranged from a central point; raphe (distinct 



canal in wall in the median region) absent. Figs. 328 and 329. 



CENTRALES* 



Fig. 328. Melosira granulata (Ehr.) Ralfs. 



Figure 328 



Fig. 329. Stephanodiscus niagarae Ehr. 



Figure 329 



1 



Figure 330 



450b Shells mostly elongate, ci- 

 gar-shaped,, boat-s h a p e d, 

 crescent-shaped, or rectan- 

 gular, with decorations bi- 

 laterally arranged from longi- 

 tudinal lines; raphe (distinct 

 longitudinal canal in the 

 wall in the median region) 

 present or absent. Figs. 330 

 to 337 PENNALES* 



Fig. 330. Fragilaria sp. 



*ln order to identify diatoms satisfactorily special techniques are required for pre- 

 paring them for microscopical study, and a descriptive key must of necessity employ 

 highly specialized terminolcgy. Space here does not permit the inclusion of all of the 

 common diatom genera which occur in fresh water (which may number 50 or more). 

 Hence a key which does not include all or most of them would be inadequate and 

 actually misleading in many instances. A few of the more common genera, especially 

 those found in the phytoplankton of lakes, are illustrated. 



186 



