ANOSTOMA. 185 



In Aiiostomrij the last whorl of the shell is suddenly 

 turned upwards, in a very singular manner, and this is 

 presumed to indicate a considerable change in the habits 

 and economy of the animal inhabitant when arriving at 

 maturity. Until then he must crawl about like others of 

 his relatives, with the spire of the shell uppermost; but, 

 during the formation of the last whorl, he must, doubtless, 

 take a sudden and reversed position, as in the full-grown 

 shell the aperture appears to be on the obverse side to that 

 in the Helix j namely, on a plane with the spire, which then 

 becomes undermost. 



The Clausilim derive their name from being furnished 

 with an elastic bony appendage attached to the columella 

 by a slender pedicle, obviously designed to close the 

 aperture. This curious appendage, unlike the operculum, 

 which uniformly adheres to the animal, is a mere addition 

 to the mouth of the shell : it is formed when the animal 

 attains his full growth, and when about to complete the 

 mouth of his shell, and answers, without doubt, some 

 important, though as yet undiscovered purpose, in the 

 animal economy. 



Vegetation, heat, and moisture, with distinctive pecu- 

 liarities of habit, affect not only the shelly coverings, but 



