144 



tHB STUDY OF SHELLS. 



the wolf fish, the inside of whose mouth is paved with 

 rounded blunt teeth as with cobblestones for crushing the 

 strong shells of such creatures. But from most creatures 

 living near them, the molluscs are very secure, the shell 

 being truly protective. 



There are several different kinds of shells, but only two 

 kinds which are at all abundant. Pupils will be able to 

 group an ordinary collection into these two natural divi- 

 sions. In one group the shell consists of a single piece, 

 which is usually, but not invariably, spiral in form, e.g. 



BEAK 



HINGE 



ANTERIOR 

 -CLOSING 

 MUSCL.E 

 MARK 



LINE 



Fig. 48. Left valve of Saucer Shell, showing markings on the inside. 



periwinkle or whelk ; in the other the complete shell con- 

 sists of two separate valves which lie right and left of the 

 animal, e.g. oyster, mussel, cockle, etc. These are appro- 

 priately termed bivalves. 



Let us take to begin with a shell of this latter type and 

 study it in detail. Any of the common bivalve shells of 

 the sea- shore will do ; a useful large example, known as 

 the saucer shell (Cyprina), is very suitable for our purpose. 



One of the characteristics of animals of the class which 

 we are dealing with, is that from the back of the animal 

 there grows out a fold of skin which is so large that it 

 covers over and conceals all the rest of the body. Quite 



