PREFACE TO THE INDEXES. 
AT the time the Mineral Conchology was commenced, so little 
was generally known of the great assistance a knowledge of fossil 
shells would prove towards the examination of the structure of 
the earth’s crust, that most collectors were very careless in ob- 
serving the relative situations from which they obtained their 
specimens; and Mr. Sowerby himself, more anxious to record the 
existence of the species as they came in his way, than to enter into 
details for which he had but indifferent means, justly considered, 
that by publishing figures with names he would at least enable 
future geologists to use terms which a reference to his work would 
render intelligible, and thus facilitate their labours and means of 
communication with each other. Mr. Farey had rendered the 
work somewhat more useful by the Supplementary Indexes which 
he furnished to the earlier volumes; but as the science of geology 
advanced he was obliged to vary his plan, and the termination of 
his life unhappily prevented him from completing the task he had 
assigned himself, of giving an improved geological arrangement to 
the whole work. 
The work was originally planned to be arranged zoologically, 
so that in the absence of an index pointing out such an arrange- 
ment it must be incomplete: this index would have been given, 
with another geologically arranged, soon after the conclusion of 
the sixth volume, but the Author of that and the one preceding 
was unwilling to adopt hastily any system which was then pro- 
posed, and even now feels that his duty is very imperfectly per- 
formed. He has adopted the system of Lamarck, as given by M. 
Deshayes, and made only a few alterations which seem to him to be 
absolutely needful: he would have made more, but was desirous 
to avoid increasing the number of systems, while he was aware 
that in all probability one more perfect than he could have planned 
would be given by a person well versed in recent shells and their 
animals, in which alone the characters necessary to be observed 
for classification can be discovered. One advantage, and that a 
considerable one, in the system he has adopted, is its being nearly 
the one followed by most modern geologists. The advancement 
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