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PREFACE. 
T 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 Arietidss 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 Zodlogy. 
Professor Langley, Secretary of the Smithsonian Institution, has shown tie 
greatest consideration and courtesy, and in undertaking the speedy pub- 
lication of this memoir after the Museum of Comparative Zovlogy 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 Tiibingen 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 Mésch at Ziirich, The late M. Barrande, 
Professor Gaudry and his assistant Dr. Fischer of the Jardin des Plantes, 
( vii ) 
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