FORMATION OF SHELLS. 219 



Various causes may occur to disturb the regu- 

 larity of the process of deposition, by which the 

 shell is enlarged in its dimensions ; at one time 

 accelerating, and at another retarding, or totally 

 arresting its growth. These irregularities are pro- 

 ductive of corresponding inequalities in the surface 

 of the shell, such as transverse lines, or strice. 

 Whenever an exuberance of materials has occa- 

 sioned a sudden expansion of growth, which has 

 again soon subsided, a projecting ridge is pro- 

 duced at the place where the margin of the mantle 

 was situated at the time this happened. This en- 

 largement 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 par- 

 ticular parts of the mantle, which become in conse- 

 quence rapidly enlarged, shooting 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 shell they con- 

 struct, being consolidated around each fleshy pro- 

 cess, must necessarily have at first the shape of a 

 tube closed at the extremity. 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 course of time, 

 every part of the cavity is obliterated, the process 

 of the shell becoming entirely solid. Such is tlie 

 origin of the many curious projecting cones or spines 



