82 AQUATIC MICROSCOPY FOR BEGINNERS. 



3. Surface roughened by rounded, pearly elevations. 



C. margarttiferum, Fig. 49. 



4. Surface roughened by small, sharp-pointed conical 



elevations. C. Brebissdnii, Fig. 50. 



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

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

 usually lie when undisturbed, 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 examined in that position, but in a moderately 

 deep cell, with considerable water, and under a low- 

 power objective, it can usually be turned over by 

 gently tapping and pressing the cover-glass with a 

 needle. 



Staurastrum is a large genus, containing about one 

 hundred and twenty species. 



In each of the succeeding figures A represents the 

 end-view of the desmid. 



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

 roughened by small elevations. St. punctuldium, 



Fig- SI- 





Fig. 51. — Staur^trum punctuldtum. Fig. 52. — Stfturdstrum furcigerum, 



2. Cell not dumb-bell shaped; with arms (a). 



a. Cell triangular in end-view, the angles toothed; 

 arms in a cluster of about three on the end of 

 the cell, their ends toothed. St.furdgerum, Fig. 52. 



