iv NORTH AMERICAN INDEX FOSSILS. 



River, instead of the Trenton. The Trenton is placed at the base 

 of the Upper Ordovicic, and regarded as representing (with the 

 Utica) a single stratigraphic unit. The Niagaran is regarded as 

 Lower Siluric, instead of Middle as formerly, while the Salinan is 

 of Middle Siluric age. The later usage of American geologists is 

 followed in considering the two main divisions of both the old Car- 

 boniferous and Cretaceous as distinct periods. For the names Coal 

 Measures or Upper Carboniferous and Lower or Sub-Carboni- 

 ferous, the names Carbonic and Mississippic are here respectively 

 used ; for the Upper and Lower Cretaceous the names Cretacic and 

 Comanchic. 



In the faunal tables (Appendix B) the species are Hsted in the 

 order in which they are described in the body of the work, and 

 each is preceded by its corresponding number, so that ready refer- 

 ence to the figures and descriptions is possible. 



The bibliography (Appendix C) while not complete, is very ex- 

 tensive including nearly all the important published works describ- 

 ing or listing fossils ; in these, descriptions and illustrations of 

 additional species as well as more detailed descriptions of the 

 species included in this work will be found ; the division mider each 

 period is for the convenience of the local student. 



The directions for collecting and preparing fossils (Appendix 

 D) are the result of the experiences of students in various parts 

 of the world. Much must be left, however, to the individual who 

 will have to adapt these methods to his particular field, or devise 

 new methods where needed. 



The glossary (Appendix E) is designed to provide short ex- 

 planations of terms for quick reference and a general index to 

 illustrations and more detailed explanations. Of the North Amer- 

 ican formation names generally only such are defined in the 

 glossary as are referred to in the body of the text; in connection 

 with this the tables of Appendix A should likewise be consulted. 



In the arrangement of the indices, which cover both volumes, 

 the same plan has been followed as in Vol. I. In addition, in the 

 index of genera the gender of each genus is indicated by the letters 

 m (mascuHne), / (feminine), or n (neuter). In the index of 

 species the gender of the genus is indicated only when a species 

 name of adjective form is followed by two or more genera of 



