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 pee 

 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. iii. 244. 

 1840.) 



N 3 



