96 



AQUATIC MICROSCOPY FOR BEGINNERS. 



Fig. 75. — Navicula cuspiddta. 



14 NAvfcuLA cuspidAta (Fig. 75), 



Valves somewhat diamond-shaped, or rhombic, 

 widest in the center, tapering 

 with straight margins to each 

 end ;' a straight line (raph6) 

 down the middle with a cen- 

 tral nodule (Fig. 75). 



15 PinnulAria (Figs. 76 and 77). 



Margins of the valves parallel, their ends and center 

 somewhat inflated; transverse ribs large and con- 

 spicuous; a line (the raphe) down the middle, with 

 a nodule at each end and at the center; frustule' 

 large. Common. P. major. Fig. 76. 



Fig. 76. — Pinnuldria major. 



Fig. 77. — Pinnularia viridis. 



2. Margins of the valves slightly convex, the endsand 

 the center not inflated; ribs large and conspicu- 

 ous; a line (the raphd) down the middle, with a 

 nodule at each end and one in the center; frustule 

 smaller than the preceding. It is named green 

 {viridis), as some one has said, probably- because 

 it is always brown. P viridis, F'ig. 77. This is 

 one of the commonest of our fresh-water diatoms. 

 Diatomists now class Pinnularia with Navicula, 

 and call these forms Navicula major and Navicula 

 viridis. 



• 16 StAURON^IS PHCENOCfiNTERON .(Fig. 78). 



Valves widest in, the middle, tapering with convexly 

 curved margins to both ends; central arid transverse 



