78 EAMBLES IN SEARCH OF SHELLS. 



All three species are plentiful in the neighbour- 

 hood of Reigate, where caperata is especially common 

 on palings and nettles. On the stems of beech trees, 

 and often concealed by ivy, may be found the rock 

 snail, Helix lainclda (PL IX., figs. 5, 6). It is about 

 the size of ericetorum, but much more solid, lens- 

 shaped, of a dark brown colour, and with a sharp 

 edge or keel round it, which distinguishes it at once 

 from all others of its kind. The crevices of rocks 

 and old walls are favourite situations for this snail ; 

 but, as it is rather inactive by day, the best time to 

 look for it is at twilight, or after a shower of rain. 

 The inappropriate name lapicida was bestowed upon 

 it by Linnffius under the erroneous impression that 

 it bored or excavated calcareous rock, as the Teredo 

 does wood.* This is one of the very few in- 



* Perforations on the under surface of projecting lime- 

 stone crags, whict H. aspersa and some other species are 

 found occupying as winter quarters, have been regarded as 

 the result of a constant resort for shelter to the same sjiot, 

 winter after winter. The erosion is believed to be due to 

 the action of the foot, aided by an acid secretion ; although 

 another theory is, that the snail works with its shell after 

 the fashion of an auger. It seems not imi^robable, however, 

 that the snails abrade the walls of these limestone cells with 

 their tongues, for the purpose of obtaining the carbonate of 

 lime. 



