146 THE STUDY OF SHELLS. 



some and very thin in others, and in shells which have 

 been dead a long time it may be all rubbed off. This sub- 

 stance, known scientifically as conchin, may also be called 

 " shell stuff." Viewed from the inside the shell shows a 

 third layer, limy but having distinctive properties. The 

 surface of this layer shows a certain amount of glossiness, 

 which in some shells produces a marked iridescence. This 

 is known as the mother-of-pearl layer, and it is from this 

 part that various useful and ornamental articles are manu- 

 factured, e.g. buttons, knife handles, etc. In the saucer 



A B 



Fig. 49. A common sea shell ( Venus gallina). 



A, the right valve ; note the beak directed forward. In B both valves are seen 

 as from the front. Under the beaks the two valves form a depressed heart-shaped 

 area, very characteristic of this type of shell. 



shell it is an opaque white. Between the two layers named 

 there is a middle limy layer. 



An examination of the shell shows that it is thicker in 

 the older parts than in the newer. This is because the 

 mantle not only adds to the shell along the edge but to its 

 inner surface also, and it is to the innermost layer that the 

 addition in thickness is made. It is owing to this property 

 of the surface of the mantle that the occurrence of pearls 

 inside shells is due. 



When bivalves are found alive, their shells are tightly 

 closed. We may appeal to the experience of the pupils, 

 some of whom have probably seen live oysters, mussels, or 

 clams. The shells are closed so tightly that it is found 



