PREFACE. 



The present volume contains descriptions and figures of all the well 

 determined species of Brachiopoda of the Upper Helderberg, Hamilton, 

 Portage and Chemung groups of New- York. Some few extra-limital species 

 have been introduced in illustration of the character of neighboring but 

 identical or analogous geological formations, and all these are from well 

 authenticated positions. In several examples of this kind, the same spe- 

 cies has subsequently been obtained within the limits of the State ; and 

 we have every reason to believe that most of those, until now known 

 only in Ohio and Canada West, will ultimately be found within the 

 State of New York. In some instances, forms previously described as 

 distinct species have been united under a single specific name. It may 

 perhaps, hereafter be necessary to pursue the same course in regard to 

 a few other examples, when the collections shall become sufl&ciently 

 extensive to enable us to determine the changes produced by physical 

 conditions, or by the geographical and geological distribution of species. 



Although the larger part of the collections used for this volume were 

 made many years since, much new material has been obtained during the 

 past few years. The number of new species, however, is not large, and 

 probably no considerable number will be added to the list of those 

 already known. These later collections have been of great importance 

 for the better determination of the geographical distribution of species, 

 and have confirmed some views on this subject advanced by the author 

 many years since. 



While the calcareous formations carry essentially the same fauna 

 from Eastern New- York throughout the length of the State, through 

 Canada West and the State of Ohio, till these limestones disappear 

 beneath the higher formations on the border of Indiana, we find impor- 

 tant changes supervening in the fauna of the sedimentary strata. 



