STRUCTURE OF SHELLS. 427 



follow the direction of the spire. On the contrary, the 

 laminae of the plate situated between the other two, form 

 concentric rings round the cone parallel with its base, and 

 cross at right angles those of the inner and outer layer. This 

 decussation of the laminae of the plates, and of the crystals 

 of the laminae themselves, adds considerably to the strength 

 of the shell, and accounts for the great difficulty that is 

 found in breaking many shells of this structure, more espe- 

 cially the Cones and Olives, in which, however, nearly the 

 whole of the strength resides in the outer whorl and in the 

 spire. 



A good illustration of this structure may be obtained by 

 examining with a pocket-glass the fractured edge of a Cone, 

 Olive, or other spiral shell, in which the extremities of the 

 laminae of the outer and inner plates, and the sides of those 

 of the central layer, or the converse, will be observed, accord- 

 ing to the direction of the fracture, the extremities of the 

 laminae showing the angles of the crystals, while their sides, 

 when closely examined, will often exhibit the crystalline 

 flakes. In order to observe the lines of cleavage, the best 

 mode of proceeding is to bruise part of a shell with a ham- 

 mer, and to examine the fragments moistened under a micro- 

 scope, until one is discovered which exhibits two laminae in 

 conjunction. The plates and their structure are also well 

 seen in the polished surfaces of shells which have been slit 

 or ground down to exhibit the internal structure of their 

 cavity. The relative thickness of the three plates varies in 

 different species ; but as far as I have yet examined, the 

 central plate is generally rather the thickest, and the outer 

 one the thinnest. The Italian cameo cutters appear to be 

 aware of this structure, and avail themselves of it in cutting 

 the cameos, the ground being always formed of the inner- 

 most layer of the three, which is also generally the most 

 transparent. 



The layers increase in thickness from their inner to their 

 outer edge, each of them being formed by successive deposi- 

 tions of thin coats of animal and calcareous matter on its 

 inner surface, until it acquires the proper thickness for the 

 shell, the outermost edge of which is very thin, and has 

 during the progress of the growth little calcareous matter, 

 but gradually passes into the periostracum. 



This accumulation of calcareous particles, deposited at 

 various times and nevertheless forming the same crystals, is 

 well illustrated in the prismatic crystalline shells. These are 

 also evidently formed of several layers, which in some in- 

 stances, as in the Pinnae, are distinctly separate from each 



