INTRODUCTION. 9 
will here be fully explained and illustrated. 
The characters of the families and genera 
will be first defined, giving their resem- 
blances and differences, and then the species 
contained in each genus will receive the lion’s 
share of attention. Here we purpose to give a 
short description of the animal and its shell, so 
‘that we may be able to distinguish the species 
from others, followed by an account of its habits, 
where it may be sought for, and how the prize 
may be secured ; in fact, all the incidents of its 
short but interesting life history will be given. 
Analytical tables, however, based on artificial 
characters, to facilitate a ready determination of 
the species and genera, will doubtless be found 
useful. 
