PREFACE. v 



of the important stratigraphic divisions, i. e. } those of wide hori- 

 zontal (geographic) but of limited vertical (stratigraphic) distri- 

 bution. Secondly those species which have a wide horizontal dis- 

 tribution even though their vertical range is also great, in other 

 words, the very common American species (e. g., Atrypa reticu- 

 laris) have been selected. Thirdly such species or representatives 

 of genera have been considered as desirable additions which, though 

 limited in distribution and in stratigraphic importance, yet furnish 

 illustration of characters needed to be understood by the student of 

 structural or anatomical palaeontology. That many species which 

 should be included have probably been omitted and others included 

 which could well be omitted, few will realize so well as the authors. 



Only a minimum of pages of the present work are devoted to a 

 discussion of the anatomical characters of the classes, for in a 

 general study of index fossils a knowledge of only the larger struc- 

 tural features is absolutely necessary. 



No descriptions of plants or of vertebrates are included. They 

 are so important and have such numerous representatives that 

 separate treatises are needed for their discussion. Certain groups, 

 as the arachnids, insects, etc., are treated only in a general way, as 

 their remains are so fragmentary as to be intelligible only to 

 specialists. Likewise other groups of limited distribution or of 

 little value as index fossils, owing to their rarity or poor preser- 

 vation, have here received only slight attention. 



The nomenclature of geologic formations advocated by the Inter- 

 national Congress of Geologists is here adopted, since this embodies 

 a uniform system of endings for terms of the same value. 



The authors will deem it a favor if users of this work will in- 

 form them of errors when detected, for errors are sure to be pres- 

 ent in a work of this character, even though carefully guarded 

 against. Especially is this the case in the distribution ranges given 

 for the species. Discoveries, which are constantly made, will sub- 

 ject this part of the work especially to modification from time 

 to time. 



In the preparation of the book the authors have enjoyed the 

 sympathetic helpfulness of many American palaeontologists who, 

 as authorities in their special fields, have rendered invaluable aid. 

 While making special acknowledgments to only a few of them, 



