10 INTRODUCTION. 



&c. usually increase in size in proportion to the 

 diameter of the aperture, and are always placed 

 with their suture or concavity towards the mouth. 

 Thus the shell is not defective, in general form, at 

 any stage of growth, each part being, at once, made 

 proportionate and entire : there is, however, a 

 limit at which the animal, most probably, and 

 certainly the shell, ceases to be capable of in- 

 crease, and it is then only that the specimen is 

 to be deemed quite complete. 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 is very evi- 

 dent to any one who has ever handled a perfect 

 shell. If, however, the margin be more turned out- 

 wards 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. 



