DIATOMACEjE. 231 



out a true rapbe ; without central and marginal nodules ; without 

 teeth, processes, or spines. 



Family 6. FragUariesB. Frustules adherent, forming a ribbon- 

 like, fan-like, or zigzag filament, or attached by a gelatinous cushion 

 or stipe ; sometimes arcuate in front, or side view. 



Epithemia, Eunotia, Fragilaria, Synedra, Diatoma. 



Family 7. Tabellarieee. Frustules with internal plates, or imper- 

 fect septa, often forming a filament. 



CKmacosphenia, Qrammatophora, Rhabdonema, TabeUaria, Stria- 

 tetta. 



Family 8. Surirellese. Frustules alate, or carinate ; frequently 

 cuneate in front view and side view. 



Nitzschia, Suriretta, Cymatopleura. 



TRIBE III. CRYPTO-RAPHIDIE<E. 



Frustules cylindrical or angular ; frequently with processes, spines, 

 teeth, or awns ; and often coherent, forming a filament. 



Family 9. Cheetocereee. Frustules mostly hyaline and armed 

 with bristles or awns, and generally coherent. 



RMzosolenia, Chcetoceros. 



Family 10. Melosirese. Frustulea cylindrical, adhering and form- 

 ing a stout filament; valves cylindrical, sometimes armed with spines. 



Melosira, Stephanopyxis. 



Family 11. Biddulphiese. Frustules adherent, forming generally 

 a zigzag filament, attached by one or two processes. 



Isthmia, Terpsinoe, Biddulphia, Hemiaulus. 



Family 12. Eupodisceee. Frustules not forming a filament ; 

 valves cylindrical, with ocelli ; often with radial ribs or furrows. 



Auliscus, Aulacodiscus, Eupoditcus. 



Family 13. Heliopelteae. Valves divided into compartments al- 

 ternately light and dark, often with marginal spines or teeth. 



Actinoptychw, HdiopeWi, Hallonyx. 



Family 14. Asterolampreee. Valves circular (rarely angular) and 

 mostly hyaline, with linear, often bifurcating, rays. 



Actinodiscus, Mastogonia, A sterolampra. 



Family 15. Coscinodisceee. Valves circular, generally with radi- 

 ating cellules, granules, or punctae ; sometimes with marginal or intra- 

 uiarginal spines or distinct ribs ; without distinct processes. 



Cydotdla, Actinocyclui, Stephanodiscus, Arachnoidiscus, Coscino- 

 discus. 



(b) Diatoms are very easily obtained for study ; it is only necessary 

 to scrape off a little of the slippery covering of submerged stones or 

 sticks to procure numerous specimens. They may be obtained also 

 from ordinary drinking water, allowing it to flow from a hydrant 

 through a filter of " Canton flannel " for an hour or so. Often appar- 



