74 MICROSCOPY FOR BEGINNERS. 



ther on in this section of the chapter, where several 

 species are noticed. 



Key to Genera of Desmids. 



1. In ribbons or narrow bands (a). 



2. Not in ribbons nor bands (d}. 



a. In a transparent, jelly-like sheath (5). 



a. Not in a jelly-like sheath (c). 



I. Each cell with two teeth on eacli narrow end. 



Didymoprium, 1. 

 5. Each cell deeply divided almost into two parts. 



Sphcerozosma, 2. 



b. Each cell without teeth and not divided. Hyalo- 



theca, 3. 



c. Cells barrel-shaped, the band not twisted. Bam- 



busina, 4. 



c. Cells not barrel-shaped, the band twisted. Desmi- 



dium, 5. 



d. Cell more or less crescent -shaped. Closterium, 



6. 



d. Cell cylindrical, spindle-shaped, hour-glass, or 

 dumb-bell shaped (/). 



d. Cell flattened, oblong, circular, or often divided 



into arms (e). 



e. Mostly circular or broadly elliptical, often cut and 



divided by slits and depressions ; marginal teeth 

 usually sharp. Micrasterias, Y. 

 e. Mostly oblong or elliptical, the margin wavy, the 

 depressions rounded. Euastrum, 8. 



