PREFACE. 



The present Memoir is the second of a series of Monographs designed 

 to make known to science the Extinct Vertebrate Life of North America. 

 In the first volume, on the Odontornithes, or Birds with Teeth, the author 

 gave the result of his investigations of that remarkable group, which he 

 discovered in the Cretaceous deposits on the Eastern slope of the Rocky 

 Mountains. 



This second Monograph contains tlie full i-ecord of a peculiar order of 

 Mammals, which the author also brought to light in the early Tertiary 

 strata of the great central plateau of the continent. 



In preparing tlie present volume, it has been the aim of the author to 

 do full justice to tlie ample material at his command, and, where possible, 

 to make the illustrations tell the main story to anatomists. The text of 

 such a Memoir may soon lose its interest, and belong to the jjast, but 

 good figures are of permanent value in all departments of Natm-al Science. 

 What is now especially needed in Palaeontology is, not long descriptions 

 of fragmentary fossils, but accurate illustrations of characteristic tvpe 

 specimens. In the fifty-six lithographic plates, and nearly two hundred 

 original woodcuts, in the present volume, it is believed that this requirement 

 is fairly met ; since all the ^nore important si^ecimens of the Dinocernta now 

 known are represented, and afleast one figm-e is given of every species. 



