The Diatoms of Nebraska 63 



c. Valves having the appearance of a stauros, either by the ab- 

 sence of striations or by the space between them. 



4. Stauroncidcae, page 81. 

 cc. Valves without any appearance of a stauros. 



d. Striations with dots not forming longitudinal lines. 



e. Striations shortened leaving a wide hyaline space along 

 the raphe. 5. Palpcbrales, page 83. 



ee. Striations extending nearly to the raphe. 



6. Perstriatae, page 83. 

 dd. Striations with dots forming straight longitudinal lines. 



7. Crassincrvcs, page 83. 

 ddd. Striations with dots forming zigzag horizontal lines. 

 c. Striations interrupted by a sulcus. 



8. Sculptac, page 86. 

 ee. Striations not interrupted by a sulcus. 



9. Seriantcae, page 87. 

 bbb. Valves with one or more marginal or submarginal sulci. 



c. Lanceolate, large, striations fine, radiate, leaving a large hj'aline 

 space around the central nodule. 



10. Fonnosae, page 88. 

 cc. Elongated, more or less triundulate ; sulcus marginal, often 



inconspicuous ; raphe surrounded by a narrow, lanceolate 

 hyaline space ; striations radiate. 



II. Limosac, page 88. 



ccc. Striations nearly perpendicular to the raphe, usually slightly 



oblique; sulcus broad, distinct. 12. Affincs, page 90. 



bbbb. Linear, no sulcus. 13. Bacillieae, page 92. 



bbbbb. Very small, structure indistinct. 14. Miiuttissimae, page 93. 



I. Pinnulariae 



A. Majores. Cells large, costae broad and heavy. 



b. Costae shortened leaving a broad hyaline space along the raphe. 



I. A^. nob His. 

 bb. Costae extending more nearly to the raphe. 



c. Inflated in the middle. 2. A'', major, 



cc. Not inflated in the middle. 3. N. viridis. 



AA. Minores. Cells of medium or small size, costae not so coarse. 

 b. Neither constricted in the middle nor undulate. 

 c. Costae heavy and distant. 



d. Linear-elliptical or elliptical, striations not extending to 



the raphe. 4. N. lata. 



dd. Linear, striations reaching nearly to the raphe. 

 e. Striations shortened in the middle. 



5. N. borealis. 



