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FORMATION OF SHELLS. 245 



has led to a sudden expansion of growth, which 

 has again soon subsided, a projecting ridge is 

 produced in the direction of the margin of the 

 mantle at the time this happens. This change 

 generally recurs at regular periods, so that these 

 ridges, or ribs, as they are often called, succeed 

 one another at equal distances along the course 

 of the spiral turns. 



It not unfrequently happens, that at different 

 periods, a sudden developement takes place in 

 particular parts of the mantle, which become in 

 consequence rapidly enlarged, shooting out into 

 long slender processes. Every part of the sur- 

 face of these processes has the power of secreting 

 and forming shell, so that the portion of shell 

 they construct, being consolidated around each 

 fleshy process, must necessarily have at first the 

 shape of a tube closed at the extremity. As 

 fresh deposits are made by the secreting sur- 

 face, 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 

 becoming entirely solid. Such is the origin of 

 the many curious projecting cones or spines 

 which several shells exhibit, and which have 

 isen periodically during their growth from 

 eir outer surface. In the Mitrex these pro- 



