The Diatoms of Nebraska 51 



13. Fragilaria 



Fragilaria Lyngb. Hydrophyt. Dan., p. 182. i8ig. 



Etym., Lat. fragilis, fragile, the filaments resembling filamentous 

 algae, but easily broken apart. 



Cells united in mostly ribbon-like, rarely zigzag chains, cells 

 symmetrical in all three planes ; girdle view rectangular, mostly 

 narrowly linear; valves linear-lanceolate or fusiform, flat, trans- 

 versely striate or with transverse rib-like beaded markings, but no 

 true ribs ; pseudoraphe present ; no nodules. 



A. Pseudoraphe narrow, scarcely visible. I. F. vircsccns. 



AA. Pseudoraphe distinct. 



b. Striations of distinct beads. 2. F. capucina. 



bb. Striations heavy with beads more or less confluent. 

 c. Small, broadly oval or cruciform. 



d. Varying from cruciform to constricted in the middle. 



3. F. construcus. 



dd. Regularly oval. 4. F. mutabilis. 



cc. Cruciform with heavy costae. 5. F. harrisonii. 



1. Fragilaria virescens Ralfs in Ann. and Mag. Nat. Hist., XII, 



pi. 2, fig. 6. 1843. 



Fragilaria vircsccns Kiitz. BacilL, p. 46, pi. 16, fig. 4. 1844. 



PI. 2, figs. 66-67. 



Cells linear or linear-elliptic with apices attenuate and often 

 rostrate-obtuse, 20-60 fx long; striations distinctly punctate, 15-18 

 in 10 fx, extending nearly across the valve, scarcely leaving a rachis. 



Hackberry Lake. • 

 Fossil : Cherry Co. 



2. Fragilaria capucina Desmaz. Crypt, de Franc, Ed. I, No. 



453- 182s. 



PI. 2, figs. 68-72. 



Valves linear, narrow with apices more or less rostrate, 23-60 |U, 

 long ; in the form called var. niesolcpta, fig. 72, constricted in the 

 middle ; margin of the valve with very distinct beads which extend 

 across the valve as delicate striations, leaving a narrow pseudo- 

 raphe, 14-15 in 10^; cells united in long band-like filaments. 



In still or slowly running water, especially common in stock tanks, also 

 fossil: Andrews, Bellevue, Cody Lake, Crawford, Culbertson, Dismal 



