210 THI'2 MECHANICAL FUNCTIONS. 



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

 JQ7 of the intermediate plate form con- 

 centric rings round the cone pa- 

 rallel to its base. Thus the fibres 

 of each layer are at right angles 

 to those of the layer which is con- 

 tiguous 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 direc- 

 tion they may be applied.* We here find that a prin- 

 ciple, which has only of late years been recognised 

 and applied to the building of ships, namely, that 

 of the diagonal arrangement of the framework, and 

 the oblique position of the timbers, is identical with 

 that which, from the beginning of creation, has been 

 acted upon by nature in the construction of shells. 

 When the form of the crystals is prismatic, the 

 fibres are short, their direction is perpendicular to 

 the surface, and the prisms are generally hexagonal. 

 This structure is observable in the Teredo gigautea 

 from Sumatra,! and also in many bivalves, such as 

 those belonging to the genera Avicula and Phma. 



* The lines indicating the direction of the fibres 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 layer, 

 there is frequently found a deposit of a hard, transparent, and appa- 

 rently homogeneous calcareous material, D. Of this latter substance 

 I shall afterwards have occasion to speak. 



t In this shell the crystalline appearance is so perfect, that when 

 some fragments were sent to England, they were mistaken for a 

 mineral production. (Home ; Lectures, i. .53.) 



