INTRODUCTION. 



23 



Varices. 

 Varices are caused by periodical rests or stoppages in the 

 growth of the shell, when the edge of the aperture thickens, 

 and renders the shell as complete as when full grown. Again, 

 after an interval, another check takes place, and another 

 thickened edge is formed, and so on in succession, until 

 the animal arrives at maturity, and the shell is full-grown. 

 The thickened edges successively forming the aperture, 

 remain visible on the outside, through all the subsequent 

 stages. When these rests take place at frequent periods, the 

 varices will of course be numerous as in Harpa and Scalaria. 

 They occur at regular or irregular distances, varying in shape 

 and other characters. When the varices occur at regular 

 intervals, and form a connected ridge from whorl to whorl up 

 the spire, they are said to be continuous, as in Ranella ; when 

 on the contrary, the varix on one whorl does not come in con- 

 tact with that on the other, they are described as discontinuous. 

 In order to distinguish a regular varix from a mere external 

 ridge, it will be sufficient to notice whether its edge overlaps 

 the external surface, and whether it resembles the open edge 

 of the aperture, which true varices do. 



Fig. 40, numerous ; 41, few, continuous ; 42, few, discontinuous. 



Aperture. 

 The aperture or opening of the spiral tube, was formerly 

 described as the mouth ; a term calculated to convev an 



