SHELLS. 41 



Child. Shells. 



Teacher. In learning the names of shells you 

 will learn also what the names imply. The 

 science which treats of shells, is called Concho- 

 logy, from the Greek word KOVM (conch) a 

 shell, and Xoyo<; (logos) a discourse. From whence 

 does the shell derive its shape and colour ? 



Child. From the animal that inhabits it. 



Teacher. What then do you think we ought 

 to study besides the shell ? 



Child. The mollusca. 



Teacher. Yes. But as we should not be 

 able to procure many of these animals, we can- 

 not pursue that branch of the science ; and we 

 will therefore follow the classification of Linnaeus, 

 which is founded on the shells. He first divides 

 shells into three great classes. Here are a large 

 number which I will arrange in three divisions, 

 and you must examine each, and observe in 

 what respect the shells I have classed together 

 resemble each other. 



Child. The shells in one set are formed of 

 one piece, in the other of two pieces, and these 

 seem to have several pieces. 



Teacher. These distinct pieces are called 

 valves. Where have you ever heard of a valve ? 



Child. A pump has a ^ valve, and steam 

 engines have valves. 



Teacher. Can you tell me what is the use of 

 the valve of a pump ? 



Child. To prevent the water from returning 

 into the pipe through which it has passed. 



