INTRODUCTION. 7 



shell. To effect this, a small quantity of mucus 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 separate 

 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 deposited on its edge, and 

 advancing beyond it necessarily adds to its extent. Thus, 

 suppose for the sake of illustration, the <?mmy~^ 

 part marked a in the diagram, fig. 4, /\/x^X 



to represent a nucleus, the cross lines (7) ; £''V^^~~\ 

 will shew the consecutive layers, which ^; : -a^^ZZZ\ 



enlarge their circle as they add to their ^sC^ ~A 



numbers. This disposition of shelly zxjS^P^ 



matter into layers is marked externally /a 



by concentric striae, ovlines of growth, 4 - imaginary cone. 



while on the inside the edges of the b [ Base 



laminae are consolidated into a kind of l - Lines of growth. 



enamel. If a perpendicular section of a solid portion of a 



shell were magnified, it would present, in many instances, an 



