14 



POPULAR CONCHOLOGY. 



the original shell, and the size merely is in- 

 creased, still preserving the same shape ; in the 

 greater number of univalve shells, however, 

 the method of enlargement is different, the 

 mantle being more active on one side of the 

 aperture than on the other : this gives rise to a 

 turbinated or spiral shell, in which each suc- 

 cessive portion of the spire, which forms an 

 entire circle, is called a whorl. A shell of 

 this nature is here shown. The 

 new formations in this genus 

 can easily be traced, each ridge 

 represented consisting of a 

 thick lip, which, at certain pe- 

 riods of its growth, the animal 

 has formed at the edge of the 

 shell then existing. These spiral 

 shells are almost always formed 

 with the whorls turning towards the right, 

 which arises from the left side of the mantle 

 being the most active, in consequence of the 

 position of the heart and blood-vessels of the 

 animal on that side * : in a few genera, however, 



* Dr. Grant makes the following observations on this 

 subject, in his Lectures on Comparative Anatomy, at the 



