In a lower horizon of the same formation, the " Sauranodon beds," were 

 found the remains of a peculiar new group of reptiles, the Sauranodontia, 

 allied to Ichthyosaurus, but without teeth. 



In the Eocene deposits of the Rocky Mountains, the writer discovered 

 a new order of huge mammals, the Dinocerata. Remains of several 

 hundred individuals were secured, and a monograph on the group will 

 follow the present memoir. In the same formation were found the remains 

 of another new order of mammals, the Tillodontia, in many respects the 

 most remarkable of any yet discovered. In the same Eocene deposits 

 were secured the first remains of fossil Primates known from North 

 America, as well as the first Cheiroptera, and Marsupialia. Abundant 

 material also was found in the same region to illustrate the genealogy of 

 the Horse, and a memoir on this subject is in cotu-se of preparation. 



To General Sherman and General Sheridan, of the United States 

 Army, my grateful acknowledgments are especially due, since without 

 their continued assistance the investigations — of which the present volume 

 is the first fruits — could not have been made. To the many other officers 

 of the Army who aided me in the field during my various explorations, 

 often in regions dangerous from hostile Indians, my sincere thanks are 

 likewise due. 



For important aid in the preparation of the present volume, I desire 

 to express my full acknowledgments to Mr. Oscar Harger, Assistant in 

 Palaeontology, in the Museum of Yale College. My thanks are also due 

 to Mr. George Bird Grinnell, Assistant in Osteology, for much valuable 

 assistance, especially while the volume was in the press. 



o. c. M. 

 Yale College, New Haven, Conn., June 16th, 1880. 



