HOW TO KNOW THE FRESH-WATER ALGAE 



125b Cells elongate-ovoid, or fusiform, or if globular, with a chloro- 

 plast parietal along the lateral walls. See Fig. 87. CHARACIUM 

 (Compare with Characiopsis, Fig. 217, a genus similar in shape, 

 belonging to the Chrysophyta in which the chromatophores are 

 yellowish-green.) 



126a (121) Cells elongate-fusiform, or rod-shaped, crescent-shaped, 

 slightly curved, or straight; several to many times longer than 

 their diameter 127 



126b Cells oval, circular (or nearly so), pyramidal, trapezoidal, or star- 

 shaped, isodiametrically angular; not more than 3 times the di- 

 ameter in length 146 



127a Cells with narrowed apices, sometimes sharply pointed. . . 1?R 



127b Cells with broadly rounded or truncate apices 133 



128a Chloroplasts 2, axial, one in either horn of a crescent-shaped cell 

 which may be only slightly curved. See Fig. 86 CLOSTERIUM 



128b Chloroplasts otherwise 129 



129a Cells decidedly fusiform, one or both poles extended into setae 

 or sharp points 130 



129b Cells not broadly fusiform 132 



130a Cells actually globular but inclosed in a fusiform sheath with 

 longitudinal ridges. Fig. 97 DESMATRACTUM 



Fig. 97. Desmafracfum bipyramidatum 

 Chod.) Pascher. 



This unique species, the only one of 

 ^\^~ the genus in this country, is rather rare 

 but seems to be widely distributed in the 

 plankton of both streams and lakes. The 

 wall is very wide and transparent, form- 

 Figure 97 i n g a sheath-like envelope. 



68 



