tv PREFACE. 



training of a stratigraphic palaeontologist is careful attention to 

 details such as is necessary in the identification of species. From 

 this arises the ability to discriminate between closely allied species 

 of one genus but of different geologic horizons, and the associa- 

 tion, in this book, of all the more important species of one genus 

 is hence not only desirable but necessary. The field geologist who 

 may use the book for the determination of his fossils and his 

 horizons, will likewise appreciate the value of a zoological arrange- 

 ment, as the stratigraphic method would require a previous knowl- 

 edge of his horizons. 



The faunal summary at the end of the work is especially addressed 

 to the needs of the student of faunas. Such faunal summaries 

 need no pictorial or graphic accompaniment. The student quali- 

 fied to use them will form mental images of the species on seeing 

 them listed, and it is only when the name stands for the species 

 m the mind of the student that he will be able to appreciate faunal 

 geography and chronology. 



Little attempt has been made at a classification and grouping 

 into divisions of less than ordinal rank. In some cases even the 

 orders have been omitted, especially where such classification is 

 based solely upon anatomical characters of the soft parts of the 

 animals and is not in full agreement with the fossil remains. The 

 authors believe that a classification should be founded upon a basis 

 of phyletic principles and a thorough appreciation of anatomical 

 details and of the relationships of hard as well as of soft parts.* 



A fair grasp of the diagnostic characteristics of a large number 

 of genera and species can not be other than helpful in the more 

 special study of genetic relationships, and such an acquaintance as 

 will be given by a study of American as well as foreign index 

 fossils, may well precede the more detailed anatomical and com- 

 parative studies which are the especial field of the professional 

 student of palaeontology and zoology. 



Much thought has been given to the selection of material from 

 the wealth of American species, it being manifestly impossible to 

 include all known species. Selection has been based on three cri- 

 teria. First to be included were the species most characteristic 



* For a discussion of the interrelationships of the hard and soft parts of 

 invertebrates and a comparison of fossil with living forms, see the forthcoming 

 introduction to the study of fossils by H. W. Shimer. 



