4 MOLLUSCA. 
has been more or less attended to by succeeding concholo- 
gists. It is certainly a very general character, and at first 
sight might be supposed worthy of forming some of the 
higher divisions. It appears but rarely in the land shells, 
more frequently in fresh water shells, and generally in the 
marine species. It does not, as yet, appear to be connect- 
ed with any peculiar organization, although it must influ- 
ence to a certain extent the economy of the animal. Were 
we, however, to employ it in higher divisions than generic 
ones, some confusion would certainly arise. It would cause 
the separation of many genera which are nearly allied, and 
even divide several natural. families. Thus, for example, 
among the porcellaneous shells, it would separate the olives 
from cones, the former being destitute of an operculum, 
while the latter possess one. ‘These two genera, however, 
belong to a natural family, the animals of both genera hav- 
ing a respiratory tube upon the head, and the eyes placed 
on the sides of the tentacula, instead of being situated, as 
in the other gasteropoda, on the tips or at the base. This 
circumstance is calculated to convince us of the necessity 
of caution in the admission of characters. These may at 
first appear to be of extensive occurrence, and well adapted 
for the formation of families, but unless they exercise some 
visible influence on the animal, they can never be employ- 
ed with propriety in a natural system, however convenient 
they may be in an artificial arrangement. Operculated 
shells may be considered as generically different from those 
which are destitute of that organ, without any injury to the - 
natural method. It would even, in many instances, be con- 
venient. 
Amongst univalve shells, considerable differences are ob- 
servable in the shape and position of the cavity of the shell. 
