GROWTH OF SHELLS. 115 



the growth of the animal, as the body extends towards 

 the mouth of the shell, its posterior end often quits the 

 first turn of the spire, and occupies a situation different 

 from that which it had originally. In these cases, the 

 cavity at the apex of the spire is filled up with solid 

 calcareous matter, not inferior in hardness to that of 

 marble. 



It sometimes happens, that, at different periods, a 

 sudden development takes place in particular parts 

 of the mantle, which, in consequence, become rapidly 

 enlarged, striking out into long slender processes. 

 Every part of the surface of these processes has the 

 power of secreting and forming shell, so that the portion 

 of it w^hich they construct, being consolidated around 

 each fleshy process, must necessarily have at first the 

 shape of a tube closed at the end. As fresh deposits 

 are made by the secreting surface, which are in the 

 interior of the tube, the internal space is gradually 

 filled up by these deposits ; the process of the mantle 

 retiring to make way for their advance towards the 

 axis of the tube. In the course of time, every part of 

 the cavity is obliterated, the process of the shell be- 

 coming entirely solid. Thus originate the many curious 

 projecting cones or spines which several shells exhibit, 

 and which have risen periodically during their growth 



