INTRODUCTORY REMARKS. 15 



account as possible of the pala?ontology of the country explored, in a 

 number of instances species have been figured and described even where 

 the specimens are not in a condition to give any clew to their generic 

 characters. This has been done, because, notwithstanding the imperfection 

 of the specimens, they can readily be identified as characteristic forms of 

 the several respective rocks. Hence, as future explorers, under more 

 fiivorable circumstances, obtain more extensive collections, it will certainl}- 

 be found necessary in perhaps not a few instances, to modify the nomen- 

 clature here provisionally adopted. In all these doubtful cases, however, 

 the doubts in regard to their generic or specific affinities have been fully 

 indicated, either by the use of a query-mark or in words. In some 

 other cases, fossils have been figured, and, as far as possible, described, 

 whei-e the species could not be certainly identified with known forms, 

 nor yet satisfactorily determined to be new. In such instances, they 

 are merely given as undetermined forms, without having any specific name 

 appended. 



In closing these remarks, I take pleasure in acknowledging my obliga- 

 tions to Professor Henry for the use of rooms, books, and specimens, as 

 well as for other facilities at the Smithsonian Institution, while preparing 

 this report. 



Smithsonian Institution, 



Washington City, D. C, August 2, 1870. 



