HOW TO KNOW THE FRESH-WATER ALGAE 



351a Envelope vase-shaped; transversely oval or pyramidal with base 



next to the substrate. Fig. 246 LAGYNION 



Fig. 246. a, Lagynion reductum 



^=^ 



Presc; b, L. triangularis var. 

 pyramidatum Presc. 



There are 3 or 4 species of this 

 genus which may be overlooked 

 in algal collections because they 

 are relatively small, often nearly 

 transparent, growing on the walls 

 of filamentous algae. The cells 

 are vase-shaped (triangular in op- 

 tical section), but this varies some- 

 what according to species. The protoplast contains a faintly pigmented, 

 yellowish chromatophore. 



351b Envelope narrowed in the posterior portion. Fig. 247 



Figure 246 



DEREPYXIS 



Figure 247 



Fig. 247. Derepyxis dispar (Stokes) Senn. 



The chief difference between this genus and 

 Lagynion (Fig. 246) is the presence of a sup- 

 porting membrane through the lorica upon 

 which the protoplast is suspended. This is the 

 only species reported from this country, occur- 

 ring as a minute epiphyte on filamentous 

 algae. 



352a (347) Cells swimming by 1 flagellum; wall impregnated with vari- 

 ously shaped silicious scales (appearing like chain armor) which 



bear bristles or needles. Fig. 248 MALLOMONAS 



Fig. 248. a, Mallomonas 



caudata Iwanoff; b, M. 



pseudocoronata Presc; 



c, M. acaroides Perty. 



These species occur in 



the open water plankton 



of mostly hard water 



lakes, frequently in abun- 



Figure 248 



dance. They are differ- 

 entiated one from the oth- 

 er by the shape and arrangement of the scales in the membrane and in 

 the arrangement of the bristles. They often occur in lakes in which 

 there is a high degree of pollution. Although motile, the single flagel- 

 lum is hardly distinguishable unless the cells are recently collected and 

 viewed under very favorable optical conditions. 



147 



