10 INTRODUCTION. 



of the aperture, and are always placed with their 

 suture or concavity toivards 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 effected 

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

 does not admit of an unexceptionable explanation, 

 but it is very evident to any one who has ever 

 handled a perfect shell. If, however, the margin 

 be more turned outwards than the common direc- 

 tion 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. 



