The Diatoms of Nebraska 115 



algae, also fossil : Andrews, Arago, Ashland, Brock, Cedar Creek, Central 

 City, Crawford, Dunning, Grand Island, Haigler, Minden, North Platte, 

 Red Cloud, Ruby, Scottsbluflf, Snake River, South Bend, Trenton, Val- 

 entine, Wann, Whitman. 



Fossil : Thedf ord. 



Belvidere, Blackpipe, Cutmeat, Little White River, Norris, Rapid City, 

 Redleaf, Rosebud, Turtle River, S. Dak. ; Devils Lake, N. Dak. ; Cheyenne, 

 W5'o. ; Ft. Collins, Colo. 



30. Cymbella 



Cyrnbella Ag. Consp., p. i. 18^0. Cynibopliora Breb. ; Gloe- 

 odictyon Ag. ; Gloeonema Ehr. ; Luniilaria Bory ; Syncyclia 

 Ehr. 



Etym., diminutive of Lat. cymba, boat, the same meaning as 

 Navicida; but Cymbella is the shape of a boat view^ed from the 

 side and Navicula is the shape of a boat viewed from above. 



Cells single, stalked (often becoming free), or enclosed in gelat- 

 inous tubes, oblong and straight in girdle view ; no interzones ; 

 valves lunate, not symmetrical ; raphe somewhat excentric, arcuate, 

 rarely straight ; central and terminal nodules present ; surface 

 transversely striate, without ribs. Chromatophore one large plate 

 the middle of which lies next to the longer girdle side of the cell, 

 the sides reflexed and lying next to both valves and reaching almost 

 to the girdle on the shorter side. In auxospore formation two 

 cells lying parallel with one another surround themselves with a 

 thick gelatinous coating and throw off the valves ; the contents of 

 each divides into two, forming four cells, these four cells unite 

 into two, each uniting with a cell from the other mother cell, 

 forming two auxospores. 



A. Ventral and dorsal margins bent in opposite directions. 

 h. Broadly lanceolate or sub-elliptic. 



c. Apices slightly produced or capitate, axial area wide, large 



forms. I. C. ehrcnbcrgii. 



cc. Apices rostrate-cuspidate, axial area narrower. 



2. C. cuspidata. 

 ccc. Apices capitate, small forms. 3. C. amphicephala. 



bb. More narro-wly lanceolate, dorsal margin much more curved than 

 the ventral. 

 c. Apices slightly attenuate. 4. C. piisilla. 



cc. Apices slightly rostrate or rostrate-capitate. 



d. Ventral margin gibbous. 5. C. Icptoccras. 



