FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 3 



In the systematic zoological part of this work, it has not seemed 

 necessary in each case to give the evidence or arguments for associa- 

 tions of upper and lower dentitions. With very few exceptions these 

 collocations are not based on actual association of upper and lower 

 jaw remains of one individual, which is very rare in this field. It is 

 based rather on occlusion, occurrence at the same locaUties, relative 

 abundance, comparison with related forms from other fields, and 

 similar indirect but usually conclusive criteria. In the few cases 

 where there is any serious doubt about the association, this fact is 

 mentioned. 



This manuscript was completed on March 15, 1936. Slight changes 

 have since been introduced, but no later general revision has been 

 made, and with few exceptions statements made are to be taken as 

 of that date. 



HISTORY OF THIS STUDY AND ACKNOWLEDGMENTS 



Work for this memoir was started in 1908, when Dr. T. W. Stanton 

 commissioned Albert C. Silberling to collect Fort Union fossils for 

 the United States Geological Survey and the United States National 

 Museum. In 1909 Dr. J. W. Gidley visited the field, where he di- 

 rected and planned further work by Silberling, and he also then under- 

 took the laboratory and office work on the collection. In view of 

 the extremely fragmentary and few fossil mammals that had been 

 collected in the Fort Union up to 1908 and the general scarcity of 

 mammals of comparable age from any formation, the magnitude to 

 which this work would grow could not have been anticipated when 

 it was started. In 1911, when Mr. Silberling finished his most inten- 

 sive work on tliis collection (to which he also added periodically until 

 1932), he had brought together one of the largest collections of Paleo- 

 cene mammals ever made. Furthermore, this collection consisted 

 almxost entirely of new species, more than half the genera were new, 

 and it greatly extended morphological and distributional knowledge 

 of the families and orders represented. The collection represented, 

 potentially, the greatest single contribution to knowledge of early 

 mammals that had ever been made. 



The very magnitude of the results achieved was embarrassing. 

 All these hundreds of specimens had to be prepared, the majority 

 of them by tediously worldng oft' the matrix grain by grain under a 

 microscope. Concurrently wdth many other duties, this was undertaken 

 by Dr. Gidley, and it occupied much of his time over a period of 12 

 years. Then the identification of the material presented great diffi- 

 culties, as it must in any Paleocene collection, for on one hand all the 

 genera and species are clearly variable and on the other the really 

 significant characters are often in such small details that it may be 

 diflficult to distinguish forms properly classified in different orders. 



