234 THE MECHANICAL FUNCTIONS. 



directions. Each layer is composed of very thin 

 plates, marked by oblique lines, 

 107 which show the direction of the 

 crystalline fibres.* The direc- 

 tion of the layers and fibres is 

 also rendered manifest by the 

 planes of cleavage, when they 

 are broken into fragments. The 

 plates of the outer and inner 

 layers are always directed from 

 the apex of the cone to its base, so as to follow 

 the direction of the spire : while, on the contrary, 

 those of the intermediate plate form concentric 

 rings round the cone parallel to its base. Thus 

 the fibres of each layer are at right angles to 

 those of the layer which is contiguous to it ; an 

 arrangement admirably calculated for giving 

 strength to the shell, by opposing a considerable 

 cohesive resistance to all forces tending to break 

 it, in whatever direction they may be applied.* 

 We here find that a principle, which has only of 

 late years been recognised and applied to the 



* These lines are shown in the diagram, Fig. 107, which re- 

 presents a longitudinal section of a shell of this kind. A is the 

 outer layer, of which the fibres pass obliquely downwards. B is 

 the middle layer, having fibres placed at right angles with the 

 former. C is the third, or inner layer, the fibres of which have 

 a direction similar to the outer layer. Within this layer there is 

 frequently found a deposit of a hard, transparent, and apparently 

 homogeneous calcareous material, D. Of this latter substance I 

 shall afterwards have occasion to speak. 



