30 Clarence J. Elmore 



ee. Without marginal spines. 



4. Coscinodiscus, page 41. 

 bb. Cells with horns (Fam. Chaetoccrataceac) . 



5. Chaetoceros, page 42. 

 bbb. Cells with short processes. 6. Biddulphia, page 43. 



^^. Valves linear, elliptical, oval, or sometimes irregular. 



b. Valves with a pseudoraphe, i.e., a clear longitudinal strip. 



c. Cells in girdle view rectangular, nearly as broad as long (Fam. 

 Tabellariaceae). 

 d. Transverse ribs of the valves when present not extending 

 into the cell cavity. 

 e. Valves with a few heavy ribs. 



7. Tctracycltts, page 44 

 ee. Valves with striations, no ribs. 



8. Striatclla, page 45 

 dd. Ribs seen in girdle view to extend deeply into the cell. 



9. Denticida, page 46- 

 cc. Cells elongated in both valve and girdle view. 



d. Cells wedge-shaped in girdle view (Fam. Mcridionaceae) 

 e. Without transverse ribs. 



10. Sccptroncis, page 47 

 ee. With transverse ribs. 



II. Meridian, page 48 

 dd. Cells rectangular in girdle view, or if of other form, the 

 rachis not median (Fam. Fragilariaccac). 

 e. Rachis median. 



/. Valves with heavy ribs. 



12. Odontidiuiu, page 49. 

 ff. Valves with striations, no ribs. 

 g. Cells short ; in filaments. 



13. Fragilaria, page 51. 

 gg. Cells longer, forming fan-like stalked clusters. 



14. Synedra, page 53. 

 ggg. Cells symmetrical, in radiating clusters. 



15. AstcrioneUa, page 57. 

 ee. Rachis near one margin. 



/. Rachis nearly on ventral margin, not interrupting 

 the striations. 16. Eunotia, page 57. 



ff. Rachis somewhat removed from the ventral mar- 

 gin, interrupting the striations. 



17. Ceratoncis, page 60. 

 bb. One or both valves with a raphe ; cells usually elongated. 



c. Rachis median or nearly so; valves not keeled (Fam. Navicti- 

 laccae). , 



