54 LESSON X. 



large proportion of such Cones as have the cha- 

 racteristics of the genus well defined.* 



Teacher. I have brought a variety of shells 

 for you to examine ; what is the point of resem- 

 hlance which unites them all in one class ? 

 Child. They are all composed of one valve. 

 Teacher. What do you therefore call them ? 

 Child. Univalves. 



Teacher. From these univalve shells select 

 those which appear to you to he similar in their 

 general appearance, and in the form of their 

 mouths. 



Children do so. 



Teacher. Explain to me what you have done. 

 Child. "We have selected those shells which 

 have the same general appearance, and a similar 

 kind of mouth. 



Teacher. What is the common point of re- 

 semblance in all the shells which I have set 

 before you ? 



Child. Their being composed of only one valve. 

 Teacher. And what are the points of dis- 

 similarity that lead you to separate this smaller 

 set from the whole group ? 



Nautilus and Argonauta are placed last because they seem 

 the connecting link between univalves and bivalves. 



* It is very important, in teaching children to draw out the 

 generic character of shells, to set before them specimens in 

 which the distinguishing features are strongly and clearly 

 marked ; their ideas will then be distinct, vivid, and perma- 

 nent. After this they may proceed to the examination of 

 shells, which, combining the qualities of different genera, 

 form the connecting links between them. 



