271 
tion of auxospores they come under the first class. If they do not 
return to the original form, but have remained distinct from it since 
Tertiary times, they have certainly earned their title to the name 
species, and should not be classed as varieties. Evolution in jdia- 
toms since their first appearance as such is not easy to trace; in 
fact, there appears to have been none. Unless we can find in them 
evidence of continued evolution, we cannot consistently recognize 
varieties among them. 
The following is a list of the species identified in these depos- 
its :* 
Abbreviations used in the list. (The data as to habitat, etc., 
are taken from De Toni, 1. c.) 
Fr. A freshwater species. 
Mar. A marine species. 
Subm. A submarine species. 
Foss. A species known elsewhere as fossil. 
Neb. A species that has been found living in Nebraska. 
M. Found in the middle layer of the Mullen deposit. 
M1. Found in the top layer of the Mullen deposit. 
M 2. Found in the bottom layer of the Mullen deposit. 
L. Found in diatomaceous limestone at Mullen. 
G. Found in Greeley county deposit. 
W. Found in Wheeler county deposit. 
T. Found in Thedford deposit. 
Amphora ovalis Kuetz. Fr. Neb. Very rare. Probably the 
form represented by var. gracilis (Ehr.) in M. L. 
Coconeis placentula Ehr. Fr. and Mar. Foss. Neb. T. M. W. 
Cymatopleura elliptica (Bréb.) W. Sm. Fr. and subm. Neb. 
Only a fragment found. M2. 
Cymatopleura solea (Bréb.) W. Sm. Fr. Neb. Not common. 
M2. 
Cymbella cistula (Hempr.) Kirchn. Fr. Neb. Common, and 
quite variable. M. W. L. G. : 
Cymbella cuspidata Kuetz. Fr. Foss. Neb. Common. Some 
Specimens measure 98 » long. W. M. T. 
* I am indebted to Prof. C. S. Boyer for examining my slides and for other sug- 
gestions, : 
