PREFACE. 



The following memoir is in continuation of the work on the Tertiary Fauna of Flor- 

 ida, which was published in Parts I. and II., Vol. 3, of these Transactions. 



In preparing the descriptive portion relating to different groups of the Pelecypoda, a 

 point was reached when it became necessary to consider the general arrangement. As 

 recent morphological and paleontological studies have thrown a new light on the relations 

 of the Pelecypoda, necessitating a revision of the earlier systems ; and as a complete revis- 

 ion is nowhere accessible in print ; it was thought that a statement of the characters chiefly 

 relied on for classifying these animals, with comparable diagnoses of the several families in 

 zoological order, would be useful for students of Paleontology and might form a proper 

 introduction to the descriptive part of this memoir. 



The data used are derived from original researches and from the literature, and the 

 characters cited in the latter have been, as far as possible, verified by comparison with 

 actual specimens from the collections of the National Jiluseum and the U. S. Geological 

 Survey 



To Dr. Chas. D. Walcott, Director, and Maj. J. W. Powell, late Director of the U. 

 S. Geological Survey, to the authorities of the Smithsonian Institution and National Museum, 

 thanks are due for assistance rendered in the prosecution of the work. 



WILLIAM HE ALLY DALL. 



