FORMATION OP SHELLS. 177 



tion dlfierent from that vvliieh it Iiad originally. In these 

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

 solid calcareous matter of a hardness not inferior to that of 

 marble. 



Such is the general form of turbinated shells. It some- 

 times happens, however, as in the Conns , that the upper 

 surface of the spiral scarcely descends 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 por- 

 tions of the spiral turns shoot downwards 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 regularity of the 

 process of deposition, by which the shell is enlarged in its 

 dimensions: at one time accelerating, and at another retard- 

 ing, 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 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 diiTcrent periods, a 

 sudden development takes place in particular parts of the 

 mantle, which become in consequence 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 construct, 

 being consolidated around each fleshy process, must neces- 

 sarily have at first the shape of a tube closed at the extre- 

 mity. As fresh dcposites are made by the secreting surface, 

 which are in the interior of the tube, the internal space is 



Vol. I. 23 



