had three toes on each foot instead of one, and many other 

 things of like kind. 



Geologists and paleontologists have been engaged for 

 three-qnarters of a century in unravelling the intricate 

 story of these strange lands and I have drawn liberally 

 from the published works of these men. My gratitude for 

 this material is hereby most gratefully acknowledged. Some 

 of the more important publications consulted are listed 

 under the heading, Bibliography. Those wishing a more 

 complete record of papers with annotations on the same 

 should consult my Bibliography of the Geology and Mining 

 Interests of the Black Hills Kegion, published as South 

 Dakota School of Mines Bulletin No. 11, 1917. I have 

 endeavored in the text or in the figures and plate descrip- 

 tions to indicate in proper way the source of material used. 



It is an especial pleasure to record here the favors ex- 

 tended by Professor Henry F. Osborn of the American 

 Museum of Natural History, by Professor W. B. Scott of 

 Princeton University, and by The Macmillan Company of 

 New York City in permitting the use of many excellent 

 figures and plates from the two great books, Osborn's Age 

 of Mammals in Europe, Asia, and North America, and 

 Scott's History of Land Mammals in the Western Hemis- 

 phere. These books deserve a large audience. They should 

 be consulted by all who wish acquaintance with mammalian 

 progress, and particularly by those interested in our White 

 River Badlands, the classic vertebrate fossil ground of 

 America. 



The subject is of absorbing interest but I have en- 

 deavored to treat it without exaggeration, sensation or 

 cheapness. The present book while following somewhat 

 closely the plan and wording of the earlier publication is 

 arranged with a little more consideration for the general 

 reader. The revised form freed from technical references 

 and faunal lists in the body of the book and with a more 

 generous use of figures and plates should be readily and 

 entirely assimilated. It is believed especially that the gen- 

 eral reader and teachers and high school students interested 

 in natural history subjects should find the information val- 

 uable and inspirational. 



CLEOPHAS C. O'HARRA. 

 November 4, 1920. 



