22 PEARLS [CH. 



layer of shell substance. This layer is known as the 

 Prismatic layer. It is of much greater thickness 

 than the periostracum, and is built up of calcareous 

 columns or prisms arranged at right-angles to the 

 surface of the shell. The prismatic layer, just like 

 the rest of the calcareous shell, consists of both 

 inorganic and organic substances. The organic part 

 forms a kind of framework upon which the calcium 

 carbonate is deposited ; the organic substance itself 

 is known as conchyolin. 



The prismatic layer also is secreted normally by 

 the cells of the mantle edge only. It cannot increase 

 in thickness after it has once been formed. The 

 prisms of which it is built are often surrounded by 

 very delicate sheaths of periostracal-like substance 

 and each prism or column is transversely striped. 

 In fact, each column looks like a little pile of 

 microscopic coins. This prismatic layer often occurs 

 in pearls. 



The third layer to be referred to is the most 

 important of all from our point of view. It is the 

 innermost layer, forming the internal surface of the 

 shell, except at the small areas where the adductor 

 muscle is attached and where there is a still more 

 internal layer of shell substance. 



It is known as the nacreous layer, and consists of 

 that valuable substance, mother-of-pearl. 



The nacreous layer differs from the layers already 



