INTRODUCTION. 



the whole. The next part, which will be the largest, will contain the Cretaceous 

 species, and the third the Tertiary. Along with the second or third part, an intro- 

 ductory chapter will be presented, giving a general sketch of the geological forma- 

 tions from which these fossils were collected ; their geographical range, lithological 

 characters, thickness, &c. ; with remarks . on their relations to formations further 

 eastward, both in this country and Europe. 



In order to make the work as useful as possible to students, descriptions of the 

 genera and families to which these fossils belong have also been added. Under 

 each family, all the genera, both recent and fossil, believed to be properly included, 

 are mentioned ; while the relations of the genera described are discussed, and at the 

 head of each generic description the synonyms, with full references, are given; like- 

 wise, when known, the etymology of the name, and the typical species of each genus. 

 The probable periods at which the several genera, as denned, were introduced, when 

 they attained their maximum development, and at what time they seem to have 

 died out, if not represented in our existing seas, are also stated. 



The portion of the work now presented, contains a larger amount of text, in 

 proportion to the number of species figured and described, than will be found in 

 the succeeding parts, in consequence of the fact that descriptions of a number of 

 genera and families are here given which it will not be necessary to repeat, the plan 

 being to refer back to these descriptions when other species belonging to any of 

 these groups come to be mentioned subsequently. 



From the references at the head of the specific descriptions it will be seen that 

 preliminary notices of most of the species have been published, from time to time, 

 under the joint names of the authors, in the Proceedings of the Philadelphia 

 Academy of Natural Sciences. Subsequently these descriptions have been almost 

 entirely rewritten, and extended, by the senior author (Mr. Meek), who has like- 

 wise prepared the accompanying descriptions of genera and families, with the 

 remarks on their relations, geological and geographical range, &c. 



The authors avail themselves of this opportunity to acknowledge their obligations 

 to Prof. Henry for the use of rooms, books, and other facilities at the Smithsonian 

 Institution, during the progress of the work: also to Prof. Dana and Prof. Agassiz, 

 for the use of a few rare works, not in the Smithsonian library. 



They are also under obligations to most of the geologists and palaeontologists of 

 the country, either for information in regard to types described by them, or respect- 

 ing the geological range of particular forms in their several fields of observation. 

 In this connection they take pleasure in mentioning the names of Mr. T. A. Conrad 

 and Mr. Wm. M. Gabb, of Philadelphia; Dr. B. F. Shumard, of St. Louis; Mr. E. 

 Billings, of the Canadian Geological Survey ; Prof. George H. Cook, State Geolo- 

 gist of New Jersey ; Mr. A. H. Worthen, State Geologist of Illinois ; and Prof. A. 

 Winchell, State Geologist of Michigan. 



tions, as examples of the more characteristic forms found in rocks of that age here, but because they 

 are either new to science, or have not been previously well illustrated in this country. Geologists, 

 however, may rest assured that they all occur directly associated with all our most common western 

 Coal Measure species. 



