The Diatoms of Nebraska 33 



Systematic Key to the Families and Genera 



A. Order Eupodiscalcs. Round Diatoms. Cells in cross section usually 



circular, less commonly polygonal, elliptical, and rarely irregular ; 



valves marked concentrically or radially by dots, areolations, lines, 



or ribs ; cells often with spines, processes, or horns. 



b. Cells without horns. Family i. Coscinodiscaceac, page T)6. 



bb. Cells with long horns or bristles. 



Family 2. Chactoccrotaccac, page 42. 

 bbb. Cells with shorter spines or processes. 



Family 3. Biddiilphiaceac, page 43. 

 AA. Order Naviculalcs. Flat Diatoms. Filaments flattened, usually frag- 

 mented into single cells ; cells narrowly elliptical to linear, less 

 commonly broadly elliptical, lunate, cuneate, or irregular : valves 

 marked pinnately or transversely by dots, areolations, lines, or 

 ribs; cells (in our species) without spines, processes, or horns. 

 b. Rachis of the valves (i.e., the line between the divergent pinnate 

 markings) evident as a narrow, unmarked strip (pseudoraphe) 

 rarely wanting; valve without a slit (raphe). 

 c. Cells usually little shorter than broad, or longer, with numerous 

 interzones, mostly united in filaments. 



Family 4. Tabcllariaceac, page 44. 



cc. Cells usually much shorter than broad (rod-shaped of older 



authors, the longer axis of the rod representing one of the 



transverse axes of the cell), often united in filaments. 



d. Cells cuneate in girdle view (i.e., valves not parallel), 



rachis median, interzones present. 



Family 5. Mcridionaccac, page 47. 

 * dd. Cells rectangular in girdle view, or if cuneate, the rachis 



not median ; interzones present or absent. 



Family 6. Fragilariaceac, page 49. 

 bb. Rachis containing an elongated slit (raphe) through the cell 

 wall. 



c. Rachis commonly median, often more or less lateral, 

 not keeled, or when keeled not punctate; interzones 

 present or absent. 



Family 7. Naviculaccac, page 60. 

 cc. Rachis lateral, less often median, punctate-keeled, 

 raphe not plainly visible. 



Family 8. Bacillariaceae, page 130. 

 bbb. Rachis evident as a narrow, unmarked strip, or keeled, valve with 

 two lateral wing keels, each enclosing a raphe. 



Family 9. SurircUaccac, page 145. 



