306 Field Columbian Museum — Geology, Vol. II. 



ACKNOWLEDGMENTS. 



The writer is indebted to the publications of Wachsmuth & Springer, 

 F. A. Bather and Stuart Weller for most of the generic descriptions 

 used in this paper, but, as they have been rearranged and adapted more 

 or less, they have not been treated as quotations. In any cases where 

 descriptions have been copied due credit has been given. 



The writer is also under obligations to Professor Weller of the Uni- 

 versity of Chicago for the use of publications, types and other speci- 

 mens from the collections of Walker Museum, which have mate- 

 rially aided in the preparation of this paper. 



The drawings illustrating the paper were made by Mr. Leon L. 

 Pray of the Museum. 



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