DESMIDS, DIATOMS, AND FRESH-WATER ALG^E. 85 



the two semi -cells evenly rounded. C. 

 Fig. 47. 



2. Surface smooth; cell about twice as long as broad, 

 the margins of each semi-cell slightly sloping tow- 

 ards the flattened ends. C. pyramidatum, Fig. 48. 



Fig. 47. Cosma- Fig. 4S. Cosma- Fig. 49. Cosma- Fig. 50. Cosroa- 

 riutn Ralfcii. rium pyrami- riiim margari- rinm Brebiss6- 



datum. tifernm. nii. 



3. Surface roughened by rounded, pearly elevations. 



C. margaritiferum, Fig. 49. 



4. Surface roughened by minute sharp points. C. Bre- 



bissonii, Fig. 50. 



12. STAURASTRUM (Figs. 51, 52, 53, 54). 



In front view, or in the position in which the desmids 

 usually lie naturally, Staurastrum resembles Cosmarium, 

 but in end view it is always angular. It is sometimes 

 rather troublesome to get Staurastrum, or indeed any 

 other desmid, tilted up on end so that it can be exam- 

 ined in that situation, but in a moderately deep cell, 

 with considerable water and a low-power objective, it 

 can usually be.turned over by gently tapping and press- 

 ing the cover-glass with a needle. 



Staurastrum is a large genus, containing about one 

 hundred and twenty species. 

 1. Cell dumb-bell shaped ; without arms ; surface rough- 



