INTRODUCTION. 7 



transparent composition than the parts subsequently produced. 

 As soon as the animal is hatched, or, in other words, leaves 

 the egg or body of the parent, of course it begins to increase 

 in size, and to require a corresponding enlargement in the 

 shell. To effect this, a small quantity of mucous substance^ 

 secreted by the mantle of the animal, is deposited on the edge 

 of the aperture. When this is dry and become sufficiently 

 hard, it is lined by a more calcareous secretion ; and these 

 together form a new layer, which is followed by others in 

 succession ; each new layer being larger than the one that 

 preceded it until the whole being complete, the full-grown 

 animal is invested with a shell commensurate with its own 

 proportions. Thus from the apex or' nucleus the formation 

 proceeds, as it were, downwards, taking the shape of the part 

 which secretes it, on which it is in a. manner moulded. 



The nucleus, or first formed portion, may for technical pur- 

 poses be considered, mathematically, as the apex of a spiral 

 cone. And here it must be observed, that whether the shell 

 consist of one or several pieces, each piece has a distinct 

 nucleus, and the process of formation is separately repeated 

 with each. The word cone is used for convenience, and its 

 meaning extended so as to include all those structures which 

 commencing at a point enlarge downwards. 



From the apex, the next layer is ^m^-^-^ 

 deposited on its edge, and advancing " >V/;^^-^^\ 

 beyond it necessarily adds to its extent. ■'^'^^^/"^'^^^-'''^k 

 Thus, suppose for the sake of illustra- %^>v^^"' \ 



tion, the part marked a in the diagram, \<nV^^^ ^ 



fig. 4, to represent a nucleus, the cross ' \^^ 



lines (Z) will shew the consecutive layers ^- ^-~-n:^_-^ 

 which enlarge their circle as they add 4. imaginary cone, 

 to their numbers. This disposal of ^' ^^^- 

 shelly matter into layers is marked i. Lines of growth. 



