PREFACE. 



TT is a common mistake to designate my classification as "embryological." 

 -*- It will be found by those who read these pages, that the whole life 

 of the individual, and all its metamorphoses, have been deemed essential 

 standards for the estimation of affinities. Even the degradational meta- 

 morphoses of old age are used as characteristics of value in the generic 

 descriptions; it is properly speaking an ontological classification. 



The researches were conducted almost wholly in Museums, because it 

 was found impracticable to study stratigraphical superposition in the field. 

 This part of the work has already been accurately done by local geolo- 

 gists, and my notes were largely made upon their collections. More 

 extended studies might have made the work more accurate than it is, 

 but this was not possible for me. 



I desire to record my deep sense of obligation to the late Prof. Louis 

 Agassiz, under whose direction my studies upon the Arietidse were begun. 

 His instruction and advice were none the less valuable because we differed 

 in theoretical views ; to him I owe the methods of observation which are 

 used in all my work. 



His son, Alexander Agassiz, has also laid scientific men in this country 

 under heavy obligations, and this essay could not have been completed or 

 published but for his sympathy, and for the liberal manner in which he 

 has sustained by large personal sacrifices the collections and the cause of 

 scientific research in the Museum of Comparative Zoology. 



Professor Langley, Secretary of the Smithsonian Institution, has shown the 

 greatest consideration and courtesy, and in undertaking the speedy pub- 

 lication of this memoir after the Museum of Comparative Zoology had 

 been obliged for want of funds to postpone its issue indefinitely, has saved 

 the results from becoming antiquated before they were made public. 



My principal studies outside of this Museum were made in the Museum 

 of Stuttgardt, and there I received unwearied attention and help from 

 Prof. Oscar Fraas, and the use of superb collections. Professor Quenstedt 

 of Tubingen gave me the benefit of much valuable information, and threw 

 open his collections without reserve, and I am indebted for similar favors 

 to Prof. Guido Sandberger at Wiirtzburg, Prof. Karl Zittel of the Museum 

 at Munich, and to Professor Mosch at Zurich. The late M. Barrande, 

 Professor Gaudry and his assistant Dr. Fischer of the Jardin des Plantes. 



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