250 POPULAR BRITISH CONCHOLOGY. 



with very short apertures, and very long many-whorled spires; 

 they are something like lengthened Bulimi, but reversed. 

 Some land-snails, as Cydostoma, have an operculum ; others, 

 as Helix, make an epiphragm : the former being constant 

 and carried on the back of the foot outside, when the ani- 

 mal is exserted ; the latter being made when wanted, and 

 then thrown away. Par more elegant and finished is the 

 contrivance with which the little Clausilia is furnished, to 

 protect itself when at rest, from the intrusion, through its 

 narrow and contracted aperture, of outward foes. The outer 

 entrance of the shell being contracted and complicated by 

 folds round the aperture, an operculum would not conveni- 

 ently accommodate itself to the opening ; so the Clausilia^ 

 retiring far within its shell, secures its retreat by a little 

 door, which lies within a groove till the animal has passed 

 it, when it springs back on its hinge and covers him. This 

 door, or clausium, is a delicate little twisted plate, attached 

 to the inner wall, or columella, of the shell by means of an 

 elastic pedicle, or hinge. When the animal wishes to move 

 out, it pushes the clausium back into a groove, where it re- 

 mains until again freed by the re-entrance of the animal. 

 It is, in fact, a spring-door, most exquisitely made and 

 beautifully finished, with the elastic spring always in good 

 repair. 



