DESMIDS, DIATOMS, AND FRESH-WATER ALGjE. 95 

 12 PLEUROSfGMA (Fig. 73.) 



Valves long S-shaped, widest in the middle and ta- 

 pering to each end, one of which curves towards the 

 right-hand side, the other towards the 

 left-hand (Fig. 73). A narrow S-shaped 

 line, the raph6, extends down the center of 

 the valve with a nodule conspicuous in 

 the middle only. There are nearly two- 

 hundred known species and varieties of 

 this genus, most of which may be recog- 

 nized by this peculiar and beautiful curva- 

 ture of the sides, the word Pleurosigma p. _pie„ro- 

 meaning S-shaped sides. The va,lves are ^'s™*- 

 very finely striated in three directions, transverse and 

 decussately oblique, the lines being remarkably close 

 together, and demanding a comparatively high-power 

 objective of excellent construction to see them. The 

 valves are therefore often used to test the good quali- 

 ties of certain objectives. To the beginner, however, 

 all.the Pleurosigmas will probably appear to be smooth. 

 The species most frequently used as a test is P. 

 anguldfum, a salt-water form. 



13 Surirella splendida (Fig. 74). 



The valves are obovate in form, with 

 transverse ribs large and conspicuous, the 

 spaces between them seeming to be lower 

 than the edges of the valves, thus giving 

 the latter the appearance of having a 

 narrow wing around the margin (Fig. 74). 

 The members of this genus are also used 

 as test-objects, the one most commonly 

 employed, Surirella gemma, beinga marine 

 species. ^'^-sp'fe^dwr*"" 



