98 MICROSCOPY FOK BEGINNERS. 



right-hand side, the other towards the left-hand (Fig. 

 73). A narrow S-shaped line extends down the centre 

 of the valve, with a nodule in the middle. 

 There are a large number of species of this 

 genus, all of which may be known by this 

 peculiar and beautiful curvature of the sides, 

 the word Pleurosigma meaning S- shaped 

 sides. The valves are very finely striated, 

 rig. 73.-pieu- 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. angu- 

 Idtum, a salt-water form. 



13. &UBIRELLA SPLENDID A (Fig. 74). 



The ribs are large and conspicuous, the spaces be- 

 tween 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 mem- 

 bers of this genus are also used as test- 

 objects, the one most commonly employed ri s . 74.1sarirei- 

 being a marine species. la 8 P 16ndida - 



14. NAVICULA CUSPIDATA (Fig. 75). 

 Valves widest in the centre, tapering with straight 



