10 
INTRODUCTION. 
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 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. 
