DESMIDS, DIATOMS, AND FRESH-WATER ALG^E. 93 



mals, winch absorb the cell contents, often leaving the 

 hard and indigestible valves colorless, but otherwise un- 

 changed. 



Key to Genera of Diatoms. 



1. Growing in bands or ribbons (a). 



2. Growing on colorless stems or in a jelly-tube (c). 



3. Growing with their concave sides attached to other 



plants (e). 



4. Free-swimming (f\ 



a. Band curved or coiled. Meridion, 1. 



a. Band zigzag ; frustules attached together by the 

 corners. Didtoma, 2. 



a. Band uneven, frustules long, narrow, rapidly slid- 

 ing on each other. Jfacilldria, 3. 



a. Band straight, or nearly so, edges even, frustules 



motionless (i). 



b. Each frustule six times as long as broad. Frage- 



Idria, 4. 



5. Each frustule twice as long as broad. Himanti- 

 dium, 5. 



c. In a narrow jelly-tube ; valves boat-shaped. En- 



cyonema, 6. 



c. On the ends of colorless stems (d). 



d. Valves boat-shaped. Cocconema, 7. 



d. Valves wedge-shaped. Gomphonema, 8. 



e. Valve six to seven times as long as broad. Epi- 



themia, 9. 



e. Valve oval, nearly as long as broad. Cocconeis, 

 10. 



