XX INTRODUCTORY REMARKS. 



For a number of years after the issue of this memoir, the writer was 

 so constantly pressed with pakeontological investigations in connection with 

 State geological surveys and General Government explorations of the 

 Western Territories, that no time was left for the rather heavy task of writing 

 out more extended descriptions of the species and genera of the much larger 

 remaining collections of Upper Missouri fossils from the later formations. 



The necessity, however, for the issue of this work became more and 

 more manifest as the several government explorations, both of our own 

 Western Territories and those of the Canadian Dominion in the adjacent 

 portions of British America, were progressing. The engraving of the 

 plates, however, being necessarily a work of time, it was urged by those 

 most interested that it ought to be commenced without waiting for the 

 entire completion of the text. About this time, it was also ascertained that 

 the whole could be published at Government expense, under the direction of 

 the Interior Department, in connection with Dr. Hay den's United States 

 Geological Survey of the Territories. Consequently, in accordance with a 

 long-established rule of the Smithsonian Institution not to publish any work 

 that could be as well brought out by any other agency, Professor Henry 

 assented to this arrangement, but expressed an entire willingness to publish 

 it, should anything occur to prevent the proposed plan from being carried 

 out. 



This method of publishing the work in connection with Dr. Hayden's 

 survey was considered all the more appropriate, because the fossils described 

 and illustrated in it were, with few exceptions, collected by his own hands, 

 and the survey under his direction is chiefly a geological work. 



In conformity with this arrangement, the contract for lithographing and 

 printing the plates was let out in 1872. As the lithography was slowly pro- 

 gressing, however, the author was still hard-pressed with work on Dr. 

 Hayden's later collections, and those of other surveys, until, in May, 1873, 

 he was suddenly prostrated by a dangerous attack of sickness, which ren- 

 dered an entire suspension of work necessary during the following summer 

 and autumn; and on the approach of cold weather the state of his health 

 was still such that a sojourn in Florida through the winter and early spring 

 months was considered necessary .* Many of the Upper Missouri type-speci- 



* Never having been provided with an assistaut, the author's labors have always necessarily been 

 entirely suspended whenever his own health failed. 



