SHELLS. 45 



rived from the Greek root <rni$ (speir,) which 

 signifies convolutions gradually increasing in dia- 

 meter, just as would be the case in a rope coiled 

 up. In the coiled rope you have the circles 

 rolled one within the other, and lying flat, or 

 being in the same plane. But if the centre whorl 

 is gradually raised above the rest, what form do 

 you obtain ? 



Child. A conical form. 



Teacher. Do you now perceive how the term 

 spire, originally derived from a word that sig- 

 nifies a set of whorls gradually increasing in 

 diameter, can be applied to a conical form ? 



Child. Yes : because when the whorls rise 

 one above another, they produce the conical form. 



Teacher. You will find the whorls of shells 

 arranged in both the ways described. When the 

 whorls are all upon the same plane, or nearly so,* 

 the spire is said to be retuse, a word derived from 

 the Latin, re, back, and tus us, beaten. Tell 

 me why this term is chosen, and pick out some 

 shells with retuse spires. 



Child. I should think the spire is called re- 

 tuse, because the whorls appear beaten back into 

 the body. 



Teacher. Exactly so ; now look at some speci- 

 mens that form quite a contrast to these retuse spires. 



Child. Here are some in which the whorls 

 gradually taper to a very fine point ; what kind 

 of spire is this ? 



* See Conus Marmoreus. Plate II. Fig. 1. 



