THE SHELL. 93 



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 sur- 

 face 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 lines the whole of 



the inner surface of the shell except a narrow 

 marginal strip. Its thickness differs in different 

 . parts and increases with the age of the mussel. It 

 consists of a number of superposed laminae laden 

 with calcareous particles. It is formed by the sur- 

 face of the body and mantle in contact with the 

 shell. 



Pearls are formed in animals closely allied to 

 the mu sel by concentric deposits of this nacreous 

 layer around parasites. 



II. DISSECTION OF THE MUSSEL. 



In dissecting the mussel, a small brush may with great 

 advantage be used to remove the mucus and to clean the several 

 parts. 



I 



