278 POPULAU BRITISH COXCHOLOGY. 



In measurement and description it is called the apex. 

 When the animal is hatched^ or leaves the ^^^^ or body of 

 the parent, it begins to enlarge its shell so as to correspond 

 with the increasing size of the body. This is done by a 

 mucous substance secreted within the mantle of the animal, 

 and deposited obliquely on the edge of the opening. This 

 is lined within by another secretion, the two forming a new 

 layer : one layer follows another till the whole is com- 

 plete. The last-formed layer forms the edge of the opening 

 into the hollow, and in the case of univalve shells this open- 

 ing is called the aperture. 



On the outside, the edges of the successive layers will 

 nearly always remain visible, being indicated by concentric 

 lines, or striae, called I'mes of groivtli. In some species the 

 layers are irregularly terminated, their edges overlapping 

 each other in such a manner that they may be easily sepa- 

 rated, and give a leafy appearance to the surface. This 

 foliated structure is seen in the common oyster. 



THE EPIDERMIS 



Is an outer covering, formed by a series of horny layers on 

 the edges of the shelly ones. The Trench name is '^ drap 

 marin," and it is sometimes more learnedly spoken of as the 



