10 INTRODUCTION. 



of the aperture, and are always placed with tlieir 

 suture or concavity touards the mouth. Thus the 

 shell is not defective, in general form, at any stage 

 of growth, each part being, at once, made propor- 

 tionate and entire : there is, however, a limit at 

 which the animal, most probably, and certainly the 

 shell, ceases to be capable of increase, and it is then 

 only that the specimen is to be deemed quite com- 

 plete. The perfecting of the aperture is not cfl'ected 

 in the same way by all of the genus, and therefore 

 does not admit of an unexceptionable explanation, 

 but is very evident to any one who has ever handled 

 a perfect shell. If, however, the margin be more 

 turned outwards than the common direction of the 

 whorls, if it be internally coloured like the upper 

 surface, finely striate or denticulate, the probability 

 is that the shell has arrived at its perfect state. 



