SHELLS. 45 



signifies convolutions gradually increasing in 

 diameter, just as would be the case in a rope 

 coiled up. In the coiled rope you have the cir- 

 cles 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 in 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 refuse, 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 

 retuse, because the whorls appear beaten back 

 into the body. 



Teacher. Exactly so; now look at some 

 specimens 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 ? 



Teacher. This very pointed spire is said to 

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



