UNIVALVES. 



103 



Saxon word meaning a round. As they successively 

 roll round one another, they gradually increase in size ; 

 and hence the set of them is called 

 the spire, from a Greek word signi- 

 fying convolutions, gradually increasing 

 in diameter, like a rope coiled up. 



If the centre whorl is gradually 

 raised above the rest^ it assumes a 

 conical form ; when the whorls are all, 

 or nearly all, on the same plane, the 

 spire is said to be retuse, from the 

 Latin word meaning beaten back, be- 

 cause they seem to be so, into the 

 body ; but if the whorls taper to a 

 fine point, the spire is said to be subu- 

 late, from the Latin term for a pointed 

 tool. The lines formed where the 

 whorls meet, is called the suture, that 

 is, the seam or joining. 



The point of the spire, or top, is 

 called the apex, d ; and the opposite, or 

 bottom part, the base, e. The opening 

 is the aperture or mouth. When the 

 shell is placed on its base, with its 

 mouth turned towards the spectator, 



