The Diatoms of Nebraska 49 



Meridion circulare Kiitz. Bacill., p. 41, pi. y, fig. 16. 1844. 



PI. 2, figs. 54-56. 



Cells oval, lanceolate, or clavate, 25-35 /^ ^oi^g' with apices 

 rounded, obtuse; about 16 striations and 3 ribs in 10 fx; pseudo- 

 raphe indistinct; girdle face cuneate, with margins somewhat un- 

 dulated by the beginning of the ribs ; cells united in a spiral fila- 

 ment. Meridion constrictnm Ralfs seems to be merely a form of 

 M. circulare, all gradations between the two being found. 



In creeks, springs, etc., also fossil : Ashland, Central City, Dunning, Gor- 

 don Creek. Halsey, Long Pine, St. Paul, Thedford. 



Fossil : Agate, Merriman, Mullen, Williams's Canyon. 



Chippewa Falls, Wis. ; Conata, Little White River, Rapid City, S. Dak. ; 

 Sully's Hill, N. Dak. ; Blackhand, Granville, New Concord, Rio Grande, 

 Rodney, Ohio. 



Family 6. FRAGILARTACEAE 



Rachis a narrow unmarked strip, no raphe ; cells generally 

 shorter than broad ; often in filaments, or sessile and nearly sep- 

 arated, adhering together at one end, forming fan-shaped colonies; 

 generally rectangular in girdle view. 



12. Odontidium 



Odontidium Riitz. Bacill.. p. 44. 1844. em. Kirchn. Alg. 

 Schles., p. 204. 18 y 8. 



Diatoma DC. Fl. Franc, II, p. 48. 180^. em. Heib. Consp. 

 Diaf. Dan., p. $j. 1863. 



Etym., Gr. nSoi'?, tooth, and i8io<;, like, from the tooth-like 

 appearance of the ribs. 



Cells united in short bands or zigzag chains which are attached 

 at the base ; valves lanceolate to linear with transverse ribs and 

 fine transverse striations, the latter interrupted centrally by an in- 

 distinct pseudoraphe ; no central nodule ; chromatophores granular. 



A. Costae delicate; cells united in zigzag chains. 



b. Broadly lanceolate. i. O. vulgarc. 



bb. Linear with ends capitate. 2. O. clougaium. 

 AA. Costae coarse; cells in short filaments. 



b. Ends rounded. 3. O. hiemale. 



bb. Ends capitate or rostrate. 4. O. anceps. 



