T]ic Diatoms of Nebraska 41 



A. Marginal dots elongated like ribs. i. S. carconciisis. 



AA. Marginal dots not elongated ; striations extending nearly to the center. 



2. S. niagarac. 



1. Stephanodiscus carconensis (Eulenst.) Grun. D'lat. Kasp. 



Mcer., p. 128. 18 y8. 



PI. I, fig. 23. 



Cell disk-shaped, plane, 30-40 /x in diameter; striations about 15 

 in 10 /t, granular, radiating, the center filled with scattered gran- 

 ules; marginal dots elongated like ribs, about 6 in io/a; margin 

 with a row of spines. 



Devils Lake, N. Dak. 



2. Stephanodiscus niagarae Ehr. in Bcr. Bed. Akad., p. 80. 



1845- 



PI. I, figs. 24-25. 



Cells disk-shaped, plane, 25-60/1, in diameter, striations granu- 

 lar, about 16 in 10 /a, radiating, extending nearly to the center, the 

 center filled with scattered grantiles; the margin with a row of 

 spines, about 4 in 10 /x. 



Benkleman. 



On rocks in creek, Cheyenne, Wyo. 



4. Coscinodiscus 



Coscinodiscus Ehr. in Ahh. Berl. Akad., p. 128. 1838. 



Syuiholophora Ehr., 1844; Odontodiscus Ehr., 1845; Hetcro- 

 stcphania Ehr.. 1854; Cestodiscus Grev., 1865; Cosiniodiscus 

 Grev., 1866; Stoschia Janisch, 1885. 

 Etym., Gr. k-octkivov, sieve, and StWos, disk. 



Cells single, circular, or sometimes elliptical; ends (valves) flat, 

 concave, or swollen ; no ribs, septa, or processes except sometimes 

 very small teeth. 



Coscinodiscus lacustris Grun. in CIcz'e & Grun. Arct. Diat.. p. 114. 

 1880. 



PI. I, fig. 26. 



Circular or sometimes broadly oval, 20-70 ix in diaineter ; puncta 

 radiating, in double rows on the margin, becoming single toward 

 the center, 4-5 rows in 10 /a on the margin, some of the rows 



