UNIVALVE MOLLUSCANS. 287 



the mucus on the sole of its foot, a portion of 

 earth and dead leaves adheres to it, which it 

 shakes off on one side ; a second portion is again 

 thus selected and deposited, and so on till it has 

 reared around itself a kind of wall of sufficient 

 height to form a cavity that will contain its shell ; 

 by turning itself round it presses against the sides 

 and renders them smooth and firm. The dome, 

 or covering, is formed in the same way : earth 

 is collected on the foot, which then is turned 

 upwards, and throws it off by exuding fresh 

 mucus; and this is repeated till a perfect roof 

 is formed. Having now completed its winter- 

 house, it draws in its foot, covering it with the 

 mantle, and opens its spiracle to draw in the air. 

 On closing this, it forms with its slime a fine 

 membrane, interposed between the mantle and 

 extraneous substances. Soon afterwards the 

 mantle secretes a large portion of very white 

 fluid over its whole surface, which instantly sets 

 uniformly, and forms a kind of solid operculum 

 like plaster of Paris, about half a line in thick- 

 ness, which accurately closes the mouth. When 

 this is become hard the animal separates the 

 mantle from it. After a time, expelling a por- 

 tion of the air it had inspired, and thus being 

 reduced in bulk, it retreats a little further into 

 the shell, and forms another leaf of mucus, and 

 continues repeating this operation till there are 

 sometimes five or six of these leaves forming 



