FORMATION OF SHELLS. 227 



operation is described by Mr. T. Bell, who repeat- 

 edly watched it, as being conducted in the following 

 manner.* A large quantity of extremely viscid 

 mucus is poured out on tlie under surface of the 

 foot, to which a layer of earth or dried leaves readily 

 adheres: this is turned on one side; and a fresh 

 secretion being thrown out, the layer of earth 

 agglutinated together by the mucus is left. The 

 animal then takes another layer of earth on the 

 bottom of the foot, turns it also to the part where he 

 intends to form the wall of his habitation, and leaves 

 it in the same manner ; repeating the process till 

 the cavity is sufficiently large, and thus making the 

 sides smooth, even, and compact. In constructing 

 the arch, or dome of the form, a similar plan is 

 followed, the foot collecting on its under surface a 

 quantity of earth ; and the animal, turning it up- 

 wards, leaves it by throwing out fresh mucus ; and 

 this is repeated until a perfect roof is formed. In 

 about an hour, or even less, after this roof is covered 

 in, the whole surface of the collar of the mantle in- 

 stantaneously pours out a considerable quantity of 

 a calcareous secretion, which is at first as fluid as 

 thick cream, but very soon acquires the consist- 

 ence of bird-lime, and is exceedingly adhesive and 

 tenacious : and in about an hour after it is poured 

 out, it has become perfectly solid, thus forming a 

 second barrier, situated more internally than the 

 first, and at a little distance from it. If at any 

 other season, while the snail is in full vigour, the 

 experiment be made of surrounding it with a freezing 

 mixture, it will immediately set about constructing 

 a covering for its protection against the cold ; and 



* Zoological Journal, i. 94. 



