STRUCTURE OF THE SHELL 79 



1. The periostracum or outer layer is thin, horny, and 



uiicalcified. To it the brownish-green colour of 

 the shell is due. It is commonly, but erroneously, 

 spoken of as the ' epidermis ' of the shell. 



In growth of the shell this layer is the first to 

 be formed, and the free edge of the shell, especially 

 round the ventral border and the posterior end, con- 

 sists of it alone. It is formed by the thickened edge 

 of the mantle-fold. 



The periostracum is reflected over the edge of the 

 shell, and lines a narrow strip of its inner surface 

 round the ventral border and at the two ends. 



2. The middle or prismatic layer forms about half the 



thickness of the shell : it is very densely calcified, 

 and consists of numerous polygonal prisms placed 

 side by side, and slightly obliquely to the surface of 

 the shell. 



Like the periostracum, it is formed by the edge 

 of the mantle. It is visible on the outer surface of the 

 shell at places where, as at the umbo, the perios- 

 tracum has been rubbed off. It can also be seen, 

 though covered by a thin layer of periostracum, 

 round the margin of the shell on the inner surface. 

 #. The inner, nacreous or pearly layer lines the whole of 

 the inner surface of the shell except the marginal 

 rim. It is about as thick as the prismatic layer, and 

 consists of a number of superposed lamina laden 

 with calcareous particles. It is formed by the whole 

 surface of the body and mantle in contact with the 

 shell. 



Pearls are formed in animals closely allied to 

 the mussel by concentric deposits of this nacreous 

 layer around foreign bodies, such as grains of sand. 



