HOW TO KNOW THE FRESH-WATER ALGAE 



324a Cells broadly fusiform, abruptly narrowed at the poles. Fig. 229. 

 PLEUROGASTER 



Fig. 229. Pleurogaster lunaris Pascher. 



The chief difference between this genus and Bumil- 

 leriopsis (Fig. 228) is the definitely fusiform shape, the 

 poles of the cells symmetrical. There are 2 species, 

 differentiated by size and variation in shape, reported 

 from the United States, but like many of the genera of 

 Chrysophyta, they are rare and never occur in pure 

 growths as do so many of the Chlorophyta and Cy- 

 anophyta. 



324b Cells narrowly fusiform, spindle-shaped or sickle-shaped. Fig. 

 230 CHLOROCLOSTER 



Fig. 230. Chloroclostei pyreniger Pascher. 



In this genus the cells are narrowly spin- 

 dle-shaped and usually are distinctly 

 curved or even sickle-shaped. They are 

 found intermingled sparingly among algal 

 mixtures from shallow water of open bogs, 

 and apparently only where the water is 

 acid. Only 1 species has been reported 

 from the United States. 



Figure 229 





Figure 230 



325a (315) Cells elongate-cylindric, with a spine at one or both poles. 326 



325b Cells short-cylindric, spherical or angular 327 



326a Cells nearly straight or only slightly bent. Fig. 231 



.CENTRITRACTUS 



Figure 231 



Fig. 231. Centiitiactus belanophorus Lenim., showing wall in 2 sections. 



There are 3 species of the genus reported from the United States, 

 differentiated by size, proportions, and shape of cell. Whereas some 

 are very short, and elliptic when young, others are very long indeed, 

 and straight or slightly curved. It is the straight form that can be used 

 to separate this genus from Ophiocytium (Fig. 219) some species of 

 which it resembles in respect to the overlapping sections of the wall, 

 color of chromatophores, etc. 



139 



