EDITOR'S PREFACE. 



Mr. Hatcher was within a few weeks of completing the manuscript of this monograph 

 when his hand was arrested by death. The typewritten manuscript, when placed in the editor's 

 hands, was found to end abruptly in the middle of a sentence in the course of the description 

 of Triceratops (Sterrliolophus) Jlabellatus (p. 147). Diligent search brought to light some more 

 pencil manuscript in Hatcher's hand, carrying the work through the systematic part as far 

 as page 157 of this monograph. This portion, referring to the genera Diceratops, Torosaurus, 

 and Nodosaurus, did not, as a consequence, have the advantage of the author's final revision. 



Part II (pp. 159-198) has been prepared by the editor, the labor being increased by the 

 preliminary necessity of mastering Hatcher's ideas and conclusions so far as known. This 

 portion has been actually compiled, whenever possible, from the author's published or written 

 opinions, the editor's purpose being to make the monograph, in so far as possible, Hatcher's 

 own. 



In the preparation of the geological portion the joint writings of Hatcher and Stanton, 

 of Stanton and Knowlton, and of L. M. Lambe have been largely used, while in Part II many 

 of the editor's own ideas and conclusions were, perforce, added where Hatcher's views could 

 not be learned from any of his writings. 



The author makes frequent reference to a chapter devoted to the revision of the genera 

 and species. This chapter (represented by the generic and specific summary in Part II), so 

 important from the systematic and nomenclatural standpoint, he unfortunately did not write, 

 and the difficult duty of preparing it has devolved upon the editor. 



The text figures were virtually completed under the author's supervision, only 12 having 

 been added by the editor. The plates were less complete, however, as several, showing the 

 comparative anatomy, had not been prepared. These have been drawn by Mrs. Lull and, it 

 is hoped, are in accordance with Mr. Hatcher's general plan. The lettering on all the plates 

 except Pis. XVIIT-XXV and XL VI was inserted under the direction of the editor. 



It should be understood that Hatcher by no means exhausted the possibilities of the sub- 

 ject, a fact which no one appreciated more fully than he. Much remains to be learned concern- 

 ing material already in the museums but as yet only partially prepared. The need of further 

 exploration and collection in the Judith River beds and their geological equivalent, the Belly 

 River of Canada, where the earliest known Ceratopsia are found, is urgent if we would have 

 an adequate understanding of the group. 



The editor has been aided in his undertaking by Professor Schuchert, of Yale University 

 Museum, by Mr. L. M. Lambe, of the Canadian Geological Survey, by Dr. W. D. Matthew, of 

 the American Museum, and by the authorities of the National Museum in Washington. To 

 Prof. Henry Fairfield Osborn especial gratitude is due for his expression of trust in placing 

 so important a task in the editor's hands and for his material aid in the loan of specimens and 

 in the final revision of the manuscript. 



Richard S. Lull. 



Paleontological Laboratory, Yale University Museum, 



July, 1906. 



