PREFACE. 



In 1896, I published with Dr. (now Professor) T. A. Jaggar, Jr., a detailed study of Melon- 

 echinus multiporus and at the same time, independently, a general study of Palaeozoic Echini. 

 These papers, which were published together in the Bulletin of the Geological Society of America, 

 were the first attempt to make a comprehensive study of this interesting group of fossil animals. 



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

 species of Palaeozoic Echini and in many cases the types. In this country similar opportuni- 

 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 types; 

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

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



The late Professor Alpheus Hyatt originated most important and novel methods of studj' 

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

 when closely associated witli 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 Incertae Sedis, the types have been studied of 69 of the 119 

 species of Palaeozoic Echini described in this memoir. Specimens of 96 of the species have been 

 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 locality, or as descrip- 

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



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