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PREFACE. 



This work is essentially a treatise on North American index 

 fossils, i. e., on those fossils best adapted for the determination 

 or geologic horizons. Its aim is to render possible in the laboratory 

 or field, ready identification of faunas and the correlation of 

 horizons. It is to some extent also a text-book of palaeontology, 

 and the needs of the student in this respect have been kept in 

 mind in its preparation; but the student of index fossils, who is 

 preparing himself for work in stratigraphic palaeontology, will 

 find this work especially planned for him; to meet his needs has 

 been the principal aim of the authors. 



The zoological rather than the stratigraphical arrangement of 

 the material seems most in harmony with this aim. Brief discus- 

 sions of the classes included in the book are followed each by its 

 own included genera. The description of each genus is followed 

 by its included species, with brief characterizations to distinguish 

 them. It is hoped that the generic keys at the beginning of each 

 class and the specific keys accompanying the fuller genera may 

 make relationships and differences more clear. The arrangement 

 of the species of any one genus is, as a rule, in chronologic succes- 

 sion from the lower to the higher formations. In some cases, as 

 in the Bryozoa, it has been possible to keep the species and genera 

 of the Palaeozoic and Mesozoic eras distinct, since in America 

 they are separated by a wide gap owing to the relative incom- 

 pleteness of the basal Mesozoic marine series. 



By this zoological arrangement attention is called to closely 

 similar genera and to still more closely similar species of one 

 genus and to the distinctions between them, in a manner not pos- 

 sible when species are grouped wholly on a stratigraphic basis. 

 Repetition of generic characters is likewise thus avoided. A trial 

 of both methods of treatment in class-room and laboratory work 

 has convinced the authors that it is the comparative method, made 

 possible by a zoological arrangement, which gives the student of 

 fossils the best grasp of the subject. The first requisite in the 



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