HOW TO KNOW THE FRESH-WATER ALGAE 



87a Cells rectangular, oval, or trapezoidal, the outer walls entire (not 

 incised); arranged to form quadrate plates in 4's or in multiples 

 of 4. Fig. 72 CRUC1GENIA 



Fig. 72 a, Crucigenia tetrapedia (Kirch.) 

 W. & G. S. West; b, C. rectangularis 

 (A. Braun) Gay. 



These cells (like Tetrastrum, Fig. 71) 

 occur in 4's but usually form multiple 

 colonies of rectangular plates. There 

 are about a dozen species in the United 

 States, differentiated by the shape of 

 the cell which may be oval, triangular 

 or elliptic in outline. 



Figure 72 



87b Cells trapezoidal, the outer free walls deeply incised, forming oval 

 or somewhat angular plates (only 4 cells present in some colonies). 

 See Fig. 70 PEDIASTRUM 



88a (84) Cell walls with spines 89 



88b Cell walls without spines 



92 



89a Cells ellipsoid, spines numerous, needle-like; cells (usually solitary) 

 arranged side by side because of interlocking spines, indefinite in 

 number. Fig. 73 FRANCEIA 



Fig. 73. Fianceia Droescheri (Lemm.) G. M. Smith. 



This and two other species occur in the euplank- 

 ton of lakes but rather rarely. The cells are solitary 

 but they may be clustered from the interlocking of 

 the needle-like spines which cover the wall. Lager- 

 heimia (Fig. 118) has a similar shape and needle- 

 like spines that are confined to the poles or to the 

 mid-region of the cell. 



Figure 73 



89b Cells ovoid or spherical; spines few (1-4), cells definitely arranged 

 and definite in number 90 



56 



