DESMIDS, DIATOMS, AND FRESH-WATER ALG^. 73 



divided on both ends almost to the middle; sheath 

 large, colorless. Five cells are shown in the figure- 

 S..piilchrum, Fig. 18. 



3. Hyaloth^ca. 



The ribbons are often very long, and the narrow 

 ends of each cell are sometimes shghtly constricted, as 

 shown in the lower part of the figure, but the depres- 

 sion is never deep enough to form teeth ; sheath color- 

 less. JI. dissUiens, Fig. 19. 





Fig. ig. — Hyalotheca dissiliens. Fig.- 20. — Bambuslna Brebiss6nii. 



4. BAMBUSfNA. 



The cells in form somewhat resemble barrels or 

 casks joined together end to end, with two narrow 

 hoop-like elevations around the middle of each. B. 

 Brebissdnii, Fig. 20. 



5. DESMfDIUM. 



The twisted appearance of the band is due to the 

 fact that the cells are triangular, as may sometimes be 

 seen when they break apart 

 and turn over on end, but the 

 three angles are not all in the 

 same line, each cell being 

 ^slightly rotated laterally.. 

 When the side of the band is looked at, it is these 

 angles that are seen like a dark oblique or zigzag line 

 traversing the ribbon. Each cell is slightly toothed 

 on both the narrow ends. Common. D. Swdrtzii, 

 Fig. 21. 



Fig. 21. — Desmidium Swartzii. 



