274 
Navicula placentula (Ehr.) Kuetz. Fr. and subm. Foss. 
Found with var. ¢amida (W.Sm.). This species shows all varia- 
tions from globose-capitate to slightly attenuate-rostrate. The 
specimens are larger than those described by De Toni (Syll. Alg. 
2: 55) and have coarser striations. The globose-capitate forms 
seem not to be described, but they evidently belong to the same 
species as the attenuate forms. Common. L. M. 
Navicula pupula Kuetz. Fr. Neb. Not very common. 
Navicula radiosa Kuetz. Fr. Foss. Neb. Var. acuta (W.Sm.) 
Grun., is found with the type, from which it scarcely differs. Rather 
common. . M2. L. 
Navicula rostrata Ehr. Fr. Foss. Neb. Not very com- 
mon. W. Navicula sphaerophora Kuetz, appears to be merely 
a form of Navzicula rostrata. M. 
Navicula serians (Bréb.) Kuetz. Fr. Foss. Neb. Rare. L. 
Navicula trinodis inflata Schultze, occurs, but is rare. Fr. Neb. 
M. 
Navicula viaidus (Nitz.) Kuetz. Fr. Foss. Neb. Not 
very common. M.L. G. 
Navicula viridula Slesvicensis (Grun.) V. H. Fr. and subm. 
Neb. Rare. M. 
Nitzschia amphibia Grun., and Nitsschia Frauenfeldi Grun., 
which seems to be merely a form of the former. Fr. Neb. M. 
GL. 
Nitsschia obtusa W.Sm. Fr. Common. L. 
Nitzschia sinuata (W. Sm.) Grun. Fr. Not very common. L. 
Nitzschia spectabilis (Ehr.) Ralfs. Subm. and Mar. Foss. 
Found in Nebraska in fresh water. Rare. L. 
Nitzschia subtilis (Kuetz.) Grun. The form represented by 
var. paleacea Grun., occurs, but is rare. Fr. and subm. Neb. M. L. 
Nitzschia vermicularis (Kuetz.) Hantzsch. Fr. Neb. Not com- 
mon. M2. 
Opephora pacifica (Grun.) Petit. Marine. This is the only 
species found in any of these deposits that has formerly been 
known as exclusively marine. Rather common. M. T. 
Stauronets anceps Ehr. Fr. and subm. Neb. Not very com- : 
mon. L. ; : 
