270 
ern Nebraska species more closely than those in the top layer do. 
The species in all of these deposits show such a general resem- 
blance to each other that it is likely that all the deposits were 
formed under the same conditions. 
In these deposits 73 distinct species were identified, besides a 
number of so-called varieties not enumerated separately. Fifty- 
nine of these, according to De Toni Syll. Alg., are exclusively fresh- 
water species, 11 are fresh or submarine, I fresh or marine, 1 ma- 
rine, and I submarine or marine. The last species, however, is 
found in Nebraska in fresh water. Only 28 of the species, accord- 
ing to De Toni,1.c.,are known as fossil, leaving 45 species that have 
not been found before as fossil. A deposit in New Jersey, how- 
ever, discovered by Dr. A. M. Edwards since the publication of 
the Sylloge Algarum, shows many of the same species that are 
found in these deposits.. There is a striking similarity between 
the species found in these deposits and those now found living in 
Nebraska. Fifty-two of the 73 species are known to be living 
within the State; and considering that only a comparatively small 
number of the living diatoms of Nebraska are known, it is likely 
that nearly all of the fossil species are now to be found living in 
the region. 
A comparison of these fossil diatoms with modern ones has- 
led me to the conclusion that the term varicty cannot be properly 
used in dealing with diatoms. The forms classed as varieties may 
be divided into two classes: (1) Conditions of some species, and 
(2) closely related species. Diatoms vary so greatly in form and 
size between one auxospore stage and the next that the same spe- 
cies may occur in many forms, all of which are but stages in the 
life-history of the organism. Many of these different conditions 
have been described as species, and have been reduced to the rank 
of varieties by later writers who saw more or less clearly their true 
relationship. But such conditions are not varieties any more than 
the prothallium of a fern or the protonema of a moss are vari- 
eties of the plants to which they belong. The other forms 
which are classed as varieties are merely closely related species. 
These fossil diatoms show the same species and the same varia- 
tions from the species that modern ones do. If these variations 
are only temporary and return to the original form by the forma- 
