66 RAMBLES IN SEAKCH OF SHELLS. 



undistinguishable. The supposed security of these 

 snails and some others, in localities where in fact 

 they are most abundant, is doubtless due to this 

 peculiar habit of ascending trees during the summer 

 months; for at this season only dead shells will 

 reward a search amongst the herbage at the 

 foot. 



In similar situations are found the close shells 

 (Clausilia), of which three out of four British 

 species have been met with in the environs of 

 London. 



These shells are worth examining carefully, on 

 account of a remarkable peculiarity in structure, 

 namely, a lid to the mouth of the shell, which closes, 

 so to say, with a spring. If the outer part of the 

 last whorl be broken off, there will be found a spoon- 

 shaped calcareous plate, or valve, attached to the 

 column of the spire by an elastic filament. When 

 the animal protrudes from its shell, this plate is 

 thrust aside, and when it withdraws it closes the 

 door, as it were, behind it. 



Clausilia biplicata (PL VIII., fig. 7) is not un- 

 common on the banks of the Thames near London ; 

 and specimens have been obtained in Hyde Park 



