The Diatoms of Nebraska 131 



one edge enclosing the raphe ; keels of the two valves on the oppo- 

 site sides of the cell except in the subdivision Hantzscliia, where 

 they are on the same side; surface punctate-striate, usually trans- 

 versely, decussate in some species. Chromatophore one large plate 

 with an entire or partial fissure and with ohe or many small 

 pyrenoids lying next to the girdle side. 



Table of Groups 

 A. Not long-rostrate. 



h. Keels of the two valves on the same side of the cell. 



I. Hantcschia, page 132. 

 bb. Keels of the two valves diagonally opposite. 



c. Valves undulate. 2. Tryblionella, page 133. 



cc. Valves not undulate. 



d. With costae extending partly across the valves or carinal 

 dots prolonged into delicate costae. 



3. Grunozvia, page 134. 

 dd. Carinal dots not prolonged into costae. 

 e. Cells more or less sigmoid. 



/. Keel straight, not inflected in the middle. 



g. Keel almost central ; girdle face with apices 



not attenuate. 4. Siginoideae, page 135. 



gg. Keel excentric; girdle face with apices slightly 



attenuate. 5. Sigmata, page 137. 



ff. Keel excentric; inflected in the middle. 



6. Obtusae, page 137. 

 ee. Cells not sigmoid. 



/. Cells arcuate. 7. Spcctabilcs, page 138. 



ff. Cells not arcuate. 



g. Girdle face constricted in the middle. 



8. Dubiae, page 138. 

 gg. Girdle face not constricted in the middle. 



h. Valves showing a longitudinal sulcus in 

 which the striations are either absent or 

 indistinct. 9. Apiculafae, page 139. 



hh. No longitudinal sulcus. 

 i. Keel excentric. 



10. Lanccolatac, page 140. 

 it. Keel not entirely excentric. 



y. Keel almost central ; striations very 

 distinct, cells united in a plate. 



II. Bacillaria. page 143. 

 jj. Keel somewhat excentric ; cells 



not united. 



