PREFACE. 



IN 1896, I published with Dr. (now Professor) T. A. Jaggar, Jr., a detail,-.! >tudy of .17 

 echinus multiporm and at the same time, independently, a general study of Palaco/oic Kchini. 

 These papers, which were published together in the Bullet in of 1 he ( illogical Society of America, 

 were the first attempt to make a comprehensive study of tins intere-ting jr rou p of fo-.-il animals. 



During a year recently spent in Europe, opportunity offered to study most of tin- European 

 species of Palaeozoic Echini and in many cases the types. In this country similar opi*>rt uni- 

 ties have been enjoyed with American species. The great liberality and kindness of all to 

 whom I applied for permission to study material is later acknowledged in detail, but I cannot 

 refrain from saying here that the freedom and liberality with which the choicest and rarest 

 material was placed in my hands and every facility offered for study at home and abroad, 

 was one of the pleasantest experiences of my life. 



The present memoir is naturally divided into three parts: first, a study of the development 

 and comparative morphology of Echini, based on the young, adult, fossil, and living ty|-: 

 second, a proposed systematic classification of Echini based on the preceding -tudie-; third, 

 a revision of Palaeozoic Echini comprising the systematic description of all known form-. 



The late Professor Alpheus Hyatt originated most important and novel method- of study 

 in Palaeontology. It is both an obligation and a privilege to say that during many jn 

 when closely associated with him as student and assistant, he impressed upon me the important 

 principles of stages in development, senescence, acceleration, and parallelism. During my 

 first sea-urchin studies I had the privilege of his ever ready sympathy and personal help. I have 

 tried to carry out this work on his lines and as he would have done it, so far as in me lay. The 

 Hyatt principles are the pith and center of the work, and I take pleasure in feeling that his 

 views worked out largely on the Mollusca are supported and strengthened by their applica- 

 tion to another group of animals. 



In revising a group, it is most important to see the types or original specimens on which 

 species have been based. Omitting Incertue Sedis, the types have been studied of t>\> ( the 1 lit 

 species of Palaeozoic Echini described in this memoir. Specimens of 96 of the specie- have boon 

 studied. These include almost all of the important forms. 



The genera and species of Palaeozoic Echini have for the most part been described in the 

 publications of geological surveys as parts of the fauna of a horizon or locali' 'rip- 



tions of a limited number of species in the proceedings of learned societies. 



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