SHELLS. 49 



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 1 



Child. A conical form. 



Teacher. Do you now perceive how the term 

 *pire, 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 each of 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, r#, back, and tusws, beaten. Tell 

 me why this term is chosen, and pick out some 

 shells with retuse spires. 



Child. I should think this spire is called re- 

 tuse, 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 

 be subulate, from the Latin subula, a pointed tool. 

 The term spire is now employed to any form re- 

 sembling that produced by whorls, which gra- 

 dually rise and decrease, though it be not oc 

 5 



