80 THE FRESHWATER MUSSEL 



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



uncalcified. 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 the 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 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 peri- 

 ostracum has been rubbed off. It can also be seen 

 round the margin of the shell on its inner surface. 



3. The inner, nacreous or pearly layer Hues the whole of 



the inner surface of the shell except the marginal 

 rim. It varies in thickness, but is in most parts 

 about as thick as the prismatic layer, and consists 

 of a number of superposed laminse laden with cal- 

 careous particles. It is formed by the 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. 



