PREFACE. 1 1 



library will compensate for any trouble undergone in turning from 

 the catalogue to the bibliography. This method of reference, with 

 some modifications, is in frequent use by scientific writers, though it 

 has probably not been employed before on so large a scale. 



Some further explanation may be made regarding the systematic 

 portion of this work. Under each genus and species there will be 

 found, first of all, a reference to the original description. This is 

 followed by the other citations, arranged according to authors in 

 alphabetic order, years of publication, and symbols. If the author 

 cited has employed a generic or specific name different from the one 

 adopted, this is thrown in parenthetically after the citation. 



The writer has arranged the classes, orders, families, and genera of 

 our fossil vertebrates according to what has seemed to him to be their 

 natural relationships. This has appeared to him to be preferable to 

 arranging them all alphabetically, although he does not overlook the 

 advantages of the latter method. On the other hand, the species of 

 each genus is here disposed in alphabetical order for the reason that 

 it will save much time in finding them, while attempts at giving them 

 a natural arrangement are often futile. 



As regards nomenclature the writer has followed the rules of the 

 American Ornithological Union, because he believes that they meet 

 the requirements of systematic work more completely and consistently 

 than those adopted by any other body of zoologists. 



The coming years will surely witness a great increase in our knowl- 

 edge of the beings which in past ages have peopled our continent. As 

 a result of discoveries and investigations many new forms will be 

 described, while again perhaps many genera and species which are now 

 accepted will be reduced to synonomy. This will, before many years, 

 make obsolete many of the names here adopted, but it is the purpose 

 of this work to facilitate just such discoveries and investigations and 

 changes. 



The writer takes pleasure in expressing his obligations to many 

 friends for assistance and encouragement in prosecuting his work on 

 this volume. Encouragement has been afforded by all, or nearly all, 

 of the vertebrate paleontologists of the United States, and as many as 

 have been applied to have given the information desired. Financial 

 assistance has been given by the Smithsonian Institution, and for this 

 acknowledgments are due, especially to its officers, Secretary S. P. 

 Langley and Assistant Secretary Richard Rathbun; and to Hon. C. D. 

 Walcott, director of the United States Geological Survey. Acknowl- 

 edgments are due to Dr. Theodore Gill for advice with regard to 

 matters pertaining to nomenclature, to scientific literature, and to classi- 

 fication. It must not, however, be inferred that he is responsible for 

 any innovations that have been made. Dr. Leonhard Stejneger, cura- 

 tor of the department of reptiles, United States National Museum, has 



