DESHIDS, DIATOMS, AND FRESH-WATER ALGyE. 99 



margins to eacli end ; a straight line down 

 the middle with a central nodule (Fig. 75). 



15. PINNULARIA (Figs. 76 and 77). 

 1. Sides of the valves parallel, the ends Fig . T J Navicu]a 

 and centre somewhat swollen ; trans- cuspidata. 

 verse ribs large and conspicuous ; a line down the 

 middle, with a nodule at each end and at the cen- 

 tre ; frustule large. Common. P. major, Fig. 76. 



Fig. 76. Piuunlana mnjor. Fig. 77. Piumilariu viridis. 



2. Sides of the valves slightly convex, the ends and the 

 centre not swollen ; ribs large and conspicuous ; a 

 line down the middle, with a nodule at each end 

 and one in the centre ; frustule smaller than the 

 preceding. It is named green (viri- 

 dis), probably because it is always 

 brown. P. viridis, Fig. 77. 



16. STAURONEIS PH<ENOCENTERON (Fig. 78). 



Yalves widest in the middle, tapering terou - 

 with curved sides to both ends ; central and transverse 

 bands smooth and conspicuous. Common (Fig. 78). 



III. FRESH-WATER ALGJS. 



The Algas often collect together and form green 

 clouds in the water or a scum-like growth on the sur- 



