ii6 Clarence J. Elmore 



dd. Ventral margin not gibbous. 



e. Raphe arcuate, striations distant. 



6. C. affinis. 

 ce. Raphe nearly straight, striations not so distant. 



7. C. naviculiformis. 

 bbb. Naviculoid, ventral margin nearly as convex as the dorsal. 



8. C. aequalis. 

 AA. Ventral and dorsal margins bent in the same direction. 



b. Terminal nodules not elongated. 



c. Striations not interrupted by a broad hyaline space on the ventral 

 side of the valve. 

 d. Raphe regularly curved toward the dorsal margin. 



e. Striations of very coarse dots. 9. C. gastroidcs. 



ec. Striations of finer dots. 



/. Large forms. 10. C. lanccolata. 



ff. Small forms. 11. C. parva. 



dd. Ends of the raphe in the middle of the valve bent toward 

 the ventral side. 12. C. cymbifonnis. 



cc. Striations interrupted by a hyaline space on the ventral side 

 of the central nodule leaving the ends of the striations as 

 isolated dots. 13. C. cistiila. 



bb. Terminal nodules elongated. 14. C. helvetica. 



1. Cymbella ehrenbergii Kiitz. Bacill., p. y^, pi. 6, fig. 11. 1844. 

 Navicula hiaequalis Ehr. Inf., pi. i^, fig. 18. 18^8. 



PI. IS, figs. 582, 583, 586, 587. 



Elliptic-lanceolate, 90-142 ;u, long, 28-38/1, wide, apices attenuate 

 or rostrate ; ventral margin nearly as convex as the dorsal ; raphe 

 nearly straight; striations 5-9 in 10 /a on the dorsal side in the 

 middle, closer elsew^here, slightly shortened, leaving a narrow space 

 along the raphe, more shortened opposite the central nodule, leav- 

 ing a rounded space. Differs from C. cuspidata mainly in size 

 and in the wider space along the raphe. 



In creeks and rivers with Cladophora and other algae, also fossil : 

 Brownlee, Dismal River, Dunning, Holt County, Seneca, Snake River, St. 

 Michael, Talmage, Thedford, Weeping Water. 



Fossil : Cherry County, Greeley County, Hay Springs, Loup City, Mullen, 

 Thedford. 



2. Cymbella cuspidata Kutz. Bacill., p. yg, pi. 5, fig. 40. 1844. 

 PI. 15, figs. 584, 585, 588. 



Broadlv linear-lanceolate, ventral side almost as convex as the 



