50 Our British Snails 



Vertigo [Sphyradium) edentula is dextral and 

 without denticles. Perhaps the most common 

 Vertigo. Partial to bracken. 



Vertigo (Isthmia) minutissima. — Dextral and 

 without denticles. Smaller, narrower, and more 

 strongly striated than edentula, but rarer. All 

 the Pupae should be examined with a magnifier. 



Balea perversa {i.e. sinistral) is a much larger 

 shell belonging to the Clausilia family. Thin, 

 dark horn-colour, semi-transparent, glossy, 7 to 8 

 whorls, local, but abundant where found. Chiefly 

 found on trees. 



Clausilia (Pirostoma) hidentata. — All our British 

 clausilias are sinistral. The clausilium (little 

 door) is an internal contrivance fastened to the 

 pillar of the shell (whereas an operculum is 

 attached to the body of a mollusc) by an elastic 

 ligament to protect it against insect enemies 

 when the animal withdraws. Bidentata has two 

 denticles, fusiform and reddish-brown, as are 

 all. Very common on walls and trees. 



Clausilia {Pirostoma) rolphii. — Rare and local. 

 Almost subterranean in habit. More coarsely 

 striated than the last. The upper whorls nearly 

 of the same breadth, forming a short cylinder. 



Clausilia {Alinda) biplicata. — Very local. 

 Chiefly on Thames willows. Larger than the 

 two former, and streaked with white. 



Clausilia {Marpessa) laminata. — Much like the 

 former, but widely distributed. Usually found 



