6 PREFACE 



Institution of Washington ; Sidney Powers, Cambridge, Massachusetts ; 

 and C. L. Dake, of the Missouri School of Mines. 



For suggestions as to the most characteristic species of the vari- 

 ous periods, credit is due to Professor G. D. Harris, for the Tertiary; 

 Messrs. T. W. Stanton and F. H. Knowlton, for the Mesozoic ; and 

 Drs. R. Ruedemann and E. M. Kindle, and Mr. L. Burling, for the 

 Paleozoic. 



The numerous block diagrams which illustrate the text were made 

 in wash rather than in line because the former are not only more 

 attractive in appearance, but because being more realistic, they are 

 more readily understood by the student. These drawings and many 

 of the diagrams which illustrate the text are the work of Mr. G. S. 

 Barkinton, to whom much credit is due. 



The writer is greatly indebted to Professors H. F. Osborn, 

 W. B. Scott, F. A. Lucas, and S. W. Williston, for permission to use 

 photographs of restorations of extinct animals, made by them or 

 under their direction, and to Professor William Bullock Clark and Dr. 

 John M. Clarke for the loan of a number of original drawings of the 

 Geological Surveys of which they are directors. 



