CLAUSILIA. 
181 
it is necessary to kill the animal in that situation 
(by drowning it), and then suffer it to dry before 
the outer lip is broken off ; and the pedicle will thus 
become fastened to the side by means of the dried 
mucus (of the body) : it may, however, at any time 
be relaxed by a little moisture, when it will instantly 
resume its elasticity, and spring from its attachment.” 
I cannot agree with Dr. Louis Pfeiffer in regard- 
ing the clausium of Clausilia as evidently analo- 
gous to the operculum of several molluscous genera.” 
First, it is not attached to the animal, as the oper- 
culum always is, but is a mere appendage to the 
mouth of the shell. Secondly, it is only formed 
when the animal has nearly reached its full growth, 
when it is about to complete the mouth of its shell, 
and not developed in the embryo of the animal while 
yet in the egg, as is the case with the operculum. 
Thirdly, the genus belongs to a group of Mollusca 
which are never operculated. 
From the above considerations I have always re- 
garded the clausium as a peculiar modification of a 
tooth or plait. I think that when its position, use, 
and mode of formation are considered, this will be 
found to be the case. If this be correct, I do not see 
how the genus Clausilia is to be separated, according 
to Dr. L. Pfeiffer’s views, from Bulimus ; and as 
Cylindrella Chemnitziana has an ovate mouth, the 
sinistral whorls of Clausilia, and incloses a rudi- 
mentary clausium, it will be necessary, if his views 
are carried out, to unite his genus Cylindrella to the 
same group. {Gray, Ann, and Mag. N. H. hi. 244. 
1840.) 
