244 THE MECHANICAL FUNCTIONS. 



descend from the apex, form by degrees a wider 

 base. During 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 cal- 

 careous matter of a hardness not inferior to that 

 of marble. 



Such is the general form of turbinated shells. 

 It sometimes happens, however, as in the Conus, 

 that the upper surface of the spiral scarcely de- 

 scends below the level of the original portion of 

 the shell, which in the former disposition of its 

 parts would have been the apex : while the 

 lower portions of the spiral turns shoot down- 

 wards so as to form a pointed process ; thus the 

 whole is still a cone, but reversed from the 

 former, the part last formed being the outer 

 surface of the cone and the circumference of 

 the apparent base, or flat surface, of which the 

 central part is the one first formed. 



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 

 productive of corresponding inequalities in the 

 surface of the shell, such as transverse lines, or 

 striae. Whenever an exuberance of materials 



