INTRODUCTION. 



The following Memoir is the first part of a work intended to contain descriptions 

 and illustrations of the invertebrate fossil remains collected by the Exploring Expe- 

 ditions under the command of Lieut, (now Maj.-Gen.) G. K. Warren, as well as 

 by Dr. Hayden and others, in the Upper Missouri country.^ It was originally pre- 

 pared with the expectation that it would form part of Lieut. Warren's official report 

 to the War Department, but circumstances having prevented the final completion 

 of the latter, with the concurrence of Lieut. Warren, the Memoir was offered to the 

 Smithsonian Institution by the authors for publication in the Smithsonian Contribu- 

 tions to Knowledge. 



Much the larger proportion of these collections being from the Cretaceous and 

 Tertiary rocks, which occupy almost the entire surface of the great area explored, 

 it was at first intended to confine the work entirely to the full illustration and de- 

 scription of the fossils of these tAvo epochs. The subsequent interesting discovery, 

 however, of Jurassic and Primordial rocks, with the intermediate Carboniferous 

 beds at the Black Hills and a few other localities, and of the Permian in Kansas, 

 rendered it necessary that some attention should also be given to the organic 

 remains of these older deposits. Yet as the Carboniferous beds, which are very 

 fossiLiferous, only occupy inconsiderable portions of the country to be reported upon, 

 while a large number of the fossils occurring in them are identical with forms already 

 published in various State and General Government Reports, and elsewhere, it has 

 not been deemed desirable to attempt to include all the laiown species from the 

 rocks of that age within the field of exploration, as this alone would require an ex- 

 tensive work. The plan adopted, therefore, is to give full descriptions and figures 

 of aU the known Tertiary, Cretaceous, Jurassic, and Primordial fossUs of this region ; 

 together with the new, and a few otherwise interesting forms, contained in the 

 collections from the Permian and Carboniferous rocks of Kansas and Southeastern 

 Nebraska.^ 



The first part of this work, now presented, includes the Primordial, Carboni- 

 ferous,^ Permian, and Jurassic species, which constitute but a smaU proportion of 



* This great extent of country was formerly known under the general name of Nebraska Territory, 

 but has been recently divided into Dakota, Nebraska, and Montana Territories. 



" No middle or upper Silurian, or Devonian beds, have yet been identified by fossils, at any 

 locality in the country explored, north of the South Pass. 



' It is proper to state, for the information of those not acquainted with the geology of the western 

 Territories, that the few carboniferous species here figured were not selected, with one or two excep- 



( vii ) 



