

PREFACE. 



In the present volume the authors present the completion of 

 their survey of those Invertebrata considered as the most important 

 fossil species of North America. The general plan of Volume I. 

 has been follovv^ed through the MoUusca, Annulosa and Echino- 

 dermata, each class having its separate structural descriptions, gen- 

 erally its key to genera, and its brief list of the more important 

 reference works, as well as a separate numbering for its genera and 

 species. In the Arthropoda, however, a departure from this method 

 was found desirable, and the subclasses rather than the classes 

 were emphasized. This was found necessary because of the great 

 differences between the subclasses. In the Malacostraca, the struc- 

 tural descriptions and literature references were brought down to 

 orders, though the numbering of genera and species is continuous 

 throughout the subclass. The same is true of the subclass Mero- 

 stomata among the Acerata, but the subclass Arachnida is treated 

 as a unit, the important species being referred to by name merely, 

 without descriptions. Each of the classes Myriopoda and Insecta 

 is treated as a whole, the species being merely mentioned by name. 

 In the class Insecta, the orders are described, with a mention and 

 illustrations of representative species. The Echinodermata having 

 such very slight relationship with any other phylum may properly 

 occupy any position except that unnatural association with the 

 Coelenterata which in early days classed these two phyla as Radiata. 



In the tables and summary of North American formations (Ap- 

 pendix A) the divisions of the earlier Palaeozoic are those else- 

 where advocated by the senior author. These and some other 

 changes have been made since the early chapters of Vol. I. (Pro- 

 tozoa to Brachiopoda inclusive) have appeared. Thus a slight 

 discrepancy will be found. The following differences are to be 

 noted. The Chazy, formerly regarded as Lower Ordovicic, is now 

 placed in the Middle Ordovicic. The Pogonip group of Nevada is 

 now regarded as Beekmantown, not Chazy. The Norman's Kill 

 shales are believed to be the approximate equivalent of the Black 



