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 Vvhich 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 bv most modern geologists. The advancement 



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