34 



Clarence J. Elmore 



Key to the Genera 



Order EUPODISCALES 



Family i. COSCINODISCACEAE 



A. Cells forming filaments ; girdle side marked. 



I. Lysigonium (Meloscira), page 36. 

 AA. Cells single ; girdle side not marked. 



b. Valve with distinct central and marginal portions. 



2. Cyclotella, page 39. 

 bb. Central and marginal portions of the valve grading into one 

 another. 

 c. With marginal spines. 



3. Stephanodtscus, page 40. 

 cc. Without marginal spines. 



4. Coscinodiscus, page 41. 



Family 2. CHAETOCEROTACEAE 



5. Chactoccros, page 42. 

 I 

 Family 3. BIDDULPHIACEAE 



6. Biddidphia, page 43. 

 Order NAVICULALES 

 Family 4. TABELLARIACEAE 



A. Transverse ribs of the valves, when present, not extending into the 

 cell cavity. 

 b. Valves with a few prominent transverse ribs. 



7. Tetracyclus, page 44. 

 bb. Valves tranversely striate. 



8. Striatclla (Tabellaria) , page 45. 

 AA. Transverse ribs of the valves extending deeply into the cell cavity. 



9. Denticula, page 46. 



Family 5. MERIDIONACEAE 



A. Valves punctate or variously punctate-striate, without transverse ribs. 



10. Sceptroncis (Opephora, Peronia), page 47. 

 AA. Valves finely striate and with transverse ribs. 



II. Meridian, page 48. 



Family 6. FRAGILARIACEAE 



A. Rachis median. 



b. Valves with transverse ribs, or if not ribbed, with a central clear 

 space (pscudoocellus). 



12. Odonlidium (including Diatoma), page 49. 



