XViii PREFACE 



n^^me^ous specimens. The commoner species often occur in great profu- 

 sion forming almost solid beds of shells many feet in thickness. In 

 general, the shells are hard and readily removed so that great numbers of 

 well-preserved specimens have been available for comparative study. 

 There have been for many years extensive collections of Maryland mate- 

 rials in several museums of the country, notably the U. S. l^ational 

 Museum, the Academy of Natural Sciences of Philadelphia, the Wagner 

 Free Institute of Science of Philadelphia, and the Johns Hopkins Uni- 

 versity. Much larger and more exhaustive collections have been made 

 in recent years by the members of the Maryland Geological Survey. All 

 of the collections, however, have been drawn upon in the present study 

 of the Miocene, The Museum of the Academy of Natural Sciences of 

 Philadelphia contains many of Dr. Conrad's types which have been most 

 important in definitely determining many of the species hitherto 

 described. 



The State Geological Survey desires to express its thanks for the aid 

 which has been rendered by the several experts who have contributed to 

 this volume; also to the U. S. Geological Survey, the Academy of Natural 

 Sciences of Philadelphia, the Wagner Free Institute of Science, the U. S. 

 National Museum and Cornell University, through Professor G. D. Har- 

 ris, which have generously allowed the use of their materials and drawings 

 and have in every way facilitated the present investigation. 



Many important suggestions have been received from Dr. W. H. Dall, 

 of the U. S. National Museum, Professor H. A. Pilsbury, of the Academy 

 of Natural Sciences of Philadelphia, and Mr. C. W. Johnson, of the 

 Boston Society of Natural History. The Survey desires especially to 

 thank Eev. Edward Huber, of Baltimore, who has generously placed at 

 the disposal of the Survey his collections of diatoms and radiolaria. 



Thanks are particularly due to the artists, the late Dr. J. C. McConnell, 

 of the U. S. Army-Medical Museum; Mr. F. von Iterson, of Princeton, 

 New Jersey; and Mr. H. C. Hunter, of the U. S. Geological Survey, for 



