The Diatoms of Nebraska 31 



d. Cells rectangular in girdle view. 



e. Cells with striations and a row of transverse septa 

 around the margins. 



18. Mastigloia, page 6r. 

 ee. Cells with striations but no septa. 

 /. Valves not keeled. 



g. Raphe straight or nearly so. 



h. No heavy lines bordering the raphe. 

 i. Cells straight in girdle view. 

 j. Central nodule round. 



k. Face of valves flat ; raphe 

 straight. 



19. Navicula, page 62. 

 kk. Face of valves convex ; raphe 

 slightly sigmoid. 

 20. Scoliopletira, page 94. 

 ;■/. Central nodule elongated trans- 

 versely forming a stauros. 



21. Stauroneis, page 95. 

 jij. Central nodule elongated longi- 

 tudinally into 4 horns. 



22. Diploneis, page 97. 

 a. Cells curved in girdle view; only one 

 valve with a raphe, 

 y. Cells elongated. 



23. Achnanthes, page 98. 

 jj. Cells nearly circular. 



24. Cocconeis, page 100. 

 hh. Raphe bordered by two heavy lines giving 

 the appearance of a double raphe. 



25. Brebissonia, page 103. 

 gg. Raphe sigmoid. 



26. Gyrosigma, page 104. 

 ff. Valves keeled. 27. Amphiprora, page 107. 



dd. Cells wedge-shaped both in girdle and valve view. 

 e. Cells straight in girdle view. 



28. Gomphonema, page 107. 

 ee. Cells curved in girdle view. 



29. Rhoicosphenia, page 114. 

 ddd. Raphe arcuate ; cells more or less moon-shaped. 

 e. Valves without transverse ribs. 



/. Raphe and central nodules not close to the margins 

 of the valves. 



