The Diatoms of Nebraska 35 



bb. Valves without transverse ribs and without central clear space. 

 c. Cells in filaments. 



13. Fragilaria, page 51.' 

 cc. Cells single or forming fan-like, stalked clusters. 



14. Synedra, page 53. 

 ccc. Cells in radiating clusters. 



15. Astcrionella, page 57. 

 AA. Rachis near one margin. 



b. Rachis nearly on ventral margin, not interrupting the striations. 



16. Eunotia, page 57. 

 bb. Rachis somewhat removed from ventral margin, interrupting the 

 striations. 



17. Ceratoneis, page 60. 



Family 7. NAVICULACEAE 



A. Valves parallel. 



b. Frustules with marginal septa. 



18. Mastigloia, page 61. 

 bb. Frustules without marginal septa. 



c. Rachis of valves not keeled. 



d. Raphe straight or nearly so. 

 e. Raphe with a simple border. 



/. Septa of interzones when present not fenestrated. 

 g. Cells straight in girdle view. 

 h. Central nodule round. 



i. Valves flat, raphe straight. 



19. Navicula, page 62. 



it. Valves convex ; raphe slightly sigmoid. 



20. Scoliopleura, page 94. 



hh. Central nodule elongated transversely, 



forming a stauros. 



♦ 21. Stauroneis, page 95. 



hhh. Central nodule elongated longitudinally 



into four horns. 



22. Diploneis, page 97. 

 gg. Cells curved in girdle view. 



23. Achnanthcs, page 98. 

 ff. Septa of interzones fenestrated ; only one valve 



with a raphe. 



24. Cocconeis, page 100. 

 ee. Raphe bordered b}' two ridges. 



25. Brebissonia, page 103. 

 dd. Raphe strongly sigmoid or arcuate. 



26. Gyrosigma (Pleurosigma) , page 104. 



