9(3 INTRODUCTION. 
This classification is better exhibited in the following 
tabular form : 
TYPE. CLASSES. 
OF.DEKS. 
I. 
Cephalopoda . . < 
■ 
Pncumobranchiata. 
Cervicobranchiata. 
SiphoDobranchiata. 
A--i|)lionobrancliiata. 
11. 
Seutibranchiata. 
MOLLUSCA. < 
Gastepopoda . . < 
III. 
Iiiferobranchiata. 
Cirrhobranchiuta. 
Monopleurobranchiatn. 
Nudibranchiata. 
Nucleobranchiata. 
Cryptobranchiata. 
r 
AcErHALA . . . < 
As the immediate subjects of this work are com- 
prised within one section of the pnemno-branchiatfe 
mollushs, namely, that containing the species living 
upon the land, it will not be necessary to give any ac- 
count of the order as a whole, beyond that contained 
in the preceding definition, except to indicate such of 
our native genera, as are by the best authorities ad- 
mitted into it, and their natural groupings into fami- 
lies. This is done in the following tabular view. It 
is proper, however, to notice, that there is much reason 
to doubt whether the family of AuricuJad<:e is truly 
pneumo-branchiate, and that the characters of the 
