DESMIDS, DIATOMS, AND FRESH-WATER ALGM.- QI 



e. Valve six to seven times as long as broad; ribs 

 conspicuous, oblique. EpithSmia, 9. 



e. Valve oval, nearly as long as broad. Cocconeis, 



10. 

 /. Valve not curved nor S-shaped (^). 



f. Valve in side view arched, the convex margins 



scalloped. Eundtia, 11. 



f. Valve long S-sbaped. Phurosigma, 12. 



/. Valve boat-shaped or obovate, the ribs sub-piur- 



allel, conspicuous. SurirMa, 13. 

 /. Valve boat-shaped, ribs none. Navicula, 14. 



g. Valve strongly ribbed transversely; a nodule at 



each end and in the center. Pinnuldria, 15. 

 g. Valve not ribbed; with a central longitudinal, 

 and a transverse smooth band, together form- 

 ing a cross. Stauron^is, 16. 



1. MERfDION CIRCULArE (Fig. 62). 



Fig. 62.— Merfdion circuldre Fig. 63.— Diitoma vulgdfe. 



Valves wedge-shaped, transverse lines indistinct; 

 bands spiral, often broken into small curved sections 

 (Fig. 62). Common. 



2. DiAtoma vulgAre (Fig. 63). 



Frustules oblong, their four angles being right- 

 angles; the band often attached to aquatic plants; 

 easily separable into its component frustules (Fig. 

 63). Common. 



