PREFACE. 



The study of the Mesozoic and Cenozoic Eehinoderniata of the United States was commenced 

 by the senior author of this paper under the auspices of the United States Geological Surve} T in 

 the autumn of 1890 and has been continued with frequent interruptions to the present time. 

 During recent years he has had associated with him, in the study of the Cenozoic forms, Dr. M. W. 

 Twitchell, formerly a graduate student at Johns Hopkins University and now assistant State 

 geologist of New Jersey, who has confined his investigations chiefly to the Clypeastrina and 

 Asternata. Several papers containing results of this study have been prepared by the senior 

 author, the most important being one entitled "The Mesozoic Echinodermata of the United 

 States," which was published in 1893 as Bulletin 97 of the United States Geological Survey. 

 Other contributions have appeared from time to time as the work has proceeded. 



Few American paleontologists have investigated the Echinodermata, and no one of them has 

 hitherto attempted a general systematic study of the Mesozoic and Cenozoic representatives of 

 this group. Much confusion has therefore existed as to the specific and generic relations of many 

 of the forms, and their geologic distribution has been in much doubt. Some of the forms are 

 very numerous, and because of their wide geographic and restricted geologic range are valuable 

 in determining questions of correlation. The revision presented in this contribution should 

 therefore be of some value in stratigraphic geology. 



No attempt has been made by the authors to modify materially the recent classifications of 

 the Echinodermata, which are based on exhaustive study of far greater and much more repre- 

 sentative materials than those afforded by American deposits. The authors have followed very 

 closely in their major classification that of F. A. Bather in ' 'The Echinodermata," in Lancaster's 

 "Treatise on zoology," and in their generic usage that of P. M. Duncan in his "Revision of the 

 genera and great groups of the Echinoidea." 



Many courtesies have been extended to the writers in connection with this study, and all the 

 large collections of the country have been put at their disposal. Among the more extensive 

 collections used have been those of the United States Geological Survey, United States National 

 Museum, Academy of Natural Sciences of Philadelphia, American Museum of Natural His- 

 tory, Wagner Free Institute of Science, Museum of Comparative Zoology of Harvard College, 

 Peabody Museum of Yale University, Cincinnati Society of Natural History, Walker Museum, 

 Carnegie Museum, Field Museum, Boston Society of Natural History, Johns Hopkins Uni- 

 versity, Williams College, University of California, Leiand Stanford Junior University, Amherst 

 College, Maryland Geological Survey, New Jersey Geological Survey, Texas Geological Survey, 

 Alabama Geological Survey, P. de Loriol, and Robert T. Jackson. 



The authors desire particularly to express then- appreciation for valuable personal aid 

 rendered by numerous paleontologists, among whom should be especially mentioned T. W. 

 Stanton, of the United States Geological Survey; J. W. Gregory, of the University of Glasgow; 

 W. H. Dall, of the United States National Museum; Robert T. Hill, formerly of the United 

 States Geological Survey; Frank Springer, of the United States National Museum; John C. 

 Merriam, of the University of California; and Ralph Arnold, until lately of the United States 

 Geological Survey. 



Most of the illustrations have been made under the direction of J. L. Ridgway, of the United 

 States Geological Survey. Many of the earlier drawings from Bulletin 97 (The Mesozoic Echino- 

 dermata of the United States), made by C. R. Keyes and H. C. Hunter, are incorporated in 

 this volume. 



