44 LESSON IX. 



Child. To the class of univalve shells. 



Teacher. I intend that you shall study the 

 univalve shells first,, because they are the most 

 simple, and their distinguishing characters well 

 marked ; but before you can describe these shells, 

 you must be well acquainted with their parts. 

 Here are some univalve shells, examine them 

 carefully ; I will give you the names for the parts, 

 as you discover them. First, tell me which 

 appear to be the principal parts. 



Child. These shells have two principal parts, 

 this which swells out, and this which is tapering. 



Teacher. The swelling part is called the 

 body,* and this which is tapering, the spire. 

 Observe how the spire is produced. 



Child. It seems formed by the rolling round 

 of a part of the shell.f 



Teacher. These parts that roll round are 

 called whorls, from an old Saxon word signifying 

 a round. What do you remark in these whorls ? 



Child. That they gradually increase in size. 



Teacher. The largest, forming the body of 

 the shell, is called the body whorl, the smallest is 

 called the first whorl. As the whorls successively 

 roll one round another, what difference is there 

 in the circles they describe ? 



Child. They gradually increase in diameter. 



Teacher. It is from this circumstance that 

 the set of whorls is called the spire, a word de- 



* See plate I. 

 f See Helix Stagnalis. Plate V. Fig. 3. 



