34 THE ELEMENTS 



of the sun, or as the index or hand of 

 a clock moves. 



" But in order to be more clearly 

 understood, let us compare the spiral 

 volutions of a shell to a common cork- 

 screw, and we shall find that whether 

 the mouth, or apex, is placed up- 

 wards, the spires will turn from the 

 upper to the lower end like a common 

 screw, which is in the same direction 

 as the index of a timepiece, and what 

 is commonly understood by a dextral 

 or right-handed screw. As a fur- 

 ther definition, such shells have their 

 aperture on the right side, when 

 examined with that end downward : 

 and is in the direction of the sun's 

 apparent motion.* 



* That different authors have described both sides of the 

 shell as the right, is very clear ; but it appears to me, Mr. Mon- 

 tagu has himself run into a mistake, concerning the right and 

 left sides of the shell ; and I have termed that side of the shell 

 with the aperture in it, when viewed in front, the left side ; 

 contrary to Mr. Montagu. It is admitted by most Concholo- 

 gists, that the apex or tip of the spire, is the top of the shell, 

 and the lower region, in winch the aperture is situate, is the 



