170 THE MECHANICAL FUNCTIONS. 



which unites the carbonate of lime is less in quantity and 

 not so evidently disposed in layers; but it is more equally 

 blended with the earthy particles, with respect to which it ap- 

 pears to perform the office of a cement, binding them strong- 

 ly together; although it has of itself but little cohesive 

 strength. The Cyprsea and the Volute are examples of por- 

 cellaneous shells. 



In shells of this kind the carbonate of lime assumes more 

 or less of a crystalline arrangement; the minute crystals be- 

 ing sometimes in the form of rhombs, and sometimes in that 

 of prisms. In the former case they are composed of three 

 distinct layers, as may be seen by making sections of any 

 of the spiral univalve shells, or simply by breaking them in 

 various directions. Each layer is composed of very thin 



plates, marked by oblique lines, which 

 show the direction of the crystalline 

 fibres.* The direction 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 

 rio-ht andes 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 



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

 er there is frequently found a desposite of a hard, transparent, and apparent- 

 ly homogeneous calcareous material, D. Of this latter substance I shall af- 

 terwards have occasion to speak. 



