12 EAMBLES IN SEARCH OF SHELLS. 



equally impossible to institute comparisons and 

 draw conclusions. 



"Without the aid of figures it is very difficult to 

 give an accurate idea of shape and size ; yet some 

 knowledge of the kind is requisite before we can 

 make any progress. 



Let us see how far a verbal description will 

 answer, if confined to species which may be regarded 

 as mere modifications of a common type or standard. 

 The idea, we believe, has not been previously sug- 

 gested.* 



Take the shell of the common garden snail (Helix 

 aspersa) as a standard. To this genus (Helix) 

 belong about five-and-twenty species out of the 

 seventy- five land shells which are generally regarded 

 as British. We shall consider the distribution and 

 rarity or otherwise of these twenty-five later. 



Now, imagine this type or standard to be in a soft 

 and plastic condition, and capable of being modelled. 

 If, holding it by the lip with the left hand, and taking 

 the apex of the spire between the finger and thumb 

 of the right hand, we slightly elongate it, what is 



* The reader may be reminded that when this chapter 

 was first published no illustrations accompanied it. 



