xlvi INTRODUCTION. [CH. 



is composed of the following materials, in every 100 

 parts : 



Organic matter 18'64 



Carbonate of lime 64 ' 96 lsi-36 



Other mineral substances, undetermined 16 '40 / 



100-00 



The structure of some shells is fibrous, and of others 

 crystalline ; and they differ considerably in the degree 

 of compactness. Nearly all the secrets of this wonderful 

 laboratory have been made known through the researches 

 of Mr. Hatchett, Dr. Carpenter, and others ; but no 

 philosopher has been able to explain why this process of 

 formation has continued from generation to generation 

 in the same uniform mould, according to the nature of 

 each species, or how the newly-born Mollusk works out, 

 with unerring and undeviating instinct, the pattern 

 which was originally designed by its Creator. Humboldt 

 justly observed that there are mysteries beyond our com- 

 prehension ; and it might be good for us that some check 

 should be put on the overweening pride of intelligence 

 in the " audax lapeti genus." 



Shape. All shells assume the form of a cone. Uni- 

 valves are generally spiral, so as to accommodate the folds 

 of the body. Even the common limpet (Patella vulgata) 

 has a distinct spire in its embryonic state, resembling 

 that of Fissurella or Emarginula. The Chitons are in 

 some respects abnormal, having several testaceous joints, 

 which overlap each other, like the plates of ancient ar- 

 mour or the scales of a fish ; but all these joints taken 

 together may be considered as forming a depressed cone 

 of greater length than that of a Patella, the lines of 

 growth on each side of the apex (which is compound in 

 Chiton) being equally symmetrical in both cases. Bi- 

 valves offer no exception to this conical law of growth, 



