28 Clarence J. Elmore 



This work has been done for very few species. Hence the 

 assigning of any form to a place as a species or that of a mere 

 form has been and still is a matter mainly of individual judgment, 

 in very few cases supported by sufficient data. 



For these reasons the term variety has been discarded in the 

 present work except as it has been necessary to use it in referring 

 to other descriptions. It is probable that future work will greatly 

 reduce the number of species of diatoms. 



DESCRIPTIONS AND DRAWINGS 



Many of the descriptions have been adapted from other works, 

 but in all cases so modified as to include any peculiarities found 

 in our local forms. The drawings are all original except that of 

 Chactoceros clmorci, this one being taken from the drawing ac- 

 companying the original description by Professor C. S. Boyer. 

 The drawings are of actual specimens and in no case has an 

 attempt been made to represent generalized or typical forms. In 

 diatoms the same species occurs in many forms, hence the large 

 number of figures given for some species. 



All of the localities where each species has been found are given. 

 This forms a fair measure of the frequency of occurrence of any 

 species. 



The writer wishes to express his indebtedness to the late Pro- 

 fessor C. E. Bessey and Professor E. H. Barbour, of the Uni- 

 versity of Nebraska, for advice and assistance in the work 

 throughout; also to Professor C. S. Boyer, of Philadelphia, for 

 the identification of several species as well as for many valuable 

 suggestions. The work of Mrs. Eleanor Barbour Cook on a par- 

 ticularly rich deposit of diatomite at Agate, Nebraska, has been 

 incorporated into this work, all of the species reported from Agate 

 being from this collection and from one sent by Mr. Harold J. 

 Cook. 



The fossil material from Greeley County was collected in 1887 

 by Mr. Russell and given to Professor Bessey, who furnished it 

 to the writer. That from other localities was furnished by Pro- 

 fessor Barbour, somfe of it having been collected by Mr. A. C. 

 Whitford, that from Thedford by Mr. J. N. Neely and Mr. J. M. 



