PREFACE. 



The present work was commenced in 1870, at which time the amonnt of 

 materials as subjects of investigation and description was comparatively small. 

 A constant accession of new materials, beyond all anticipation, has greatly 

 extended the work. This will account for the apparent want of system in 

 the arrangement or proper collocation of the subjects of many of the plates. 



The interest excited by the numerous discoveries of vertebrate fossils in 

 the Western States and Territories has led to the recent explorations of Pro- 

 fessors Marsh and Cope, both of whom have obtained rich collections. The 

 investigations and descriptions, by these gentlemen, of some of the fossils 

 from the same localities, have been so nearly contemporary with my own, that, 

 from want of the opportunity of comparison of specimens, we have no doubt 

 in some cases described the same things under different names, and thus pro- 

 duced some confusion, which can only be corrected in future. 



My investigations, in many instances, may appear not so complete as would 

 be desirable, and my excuse for not doing the work more thoroughly is the 

 limited time allowed for the purpose and the little leisure I have had in the 

 intervals of other and necessary professional engagements. 



