54 



BRITISH MOLLUSKS. 



Helix aspersa. 



Genus IV. HELIX, Linnceus. 



Animal ; sometimes stout, sometimes slender, bearing an orbicularly 

 coiled shell, varying considerably in general character ; mantle 

 not reflected over the shell, head with the upper pair of tentacles 

 rather long, the lower short, foot lanceolate posteriorly, never 

 truncate, minute palate teeth serrated. 

 Shell ; mostly umbiHcated, globose, varying to depressly orbicular, 

 sometimes sobd, gaily coloured, more frequently thin and horny, 

 spire only moderately exserted, whorls varying from about four 

 to six, mostly striated in the direction parallel with the lines of 

 growth, aperture more or less lunar-rounded, lip in the solid 

 species reflected, in the thin horny species simple. 

 Compared with Zonites, the animal of Helix differs in being lan- 

 ceolate and never truncate at the tail extremity, and in not reflect- 

 ing the edge of the mantle over the shell. The polished surface im- 

 parted by this action to the shell of Zonites, is never seen in Helix, 

 so that the genera are distinguished from each other by good concho- 

 logical characters, resulting from a corresponding difference in the 

 animal. In neither genus has the animal any operculum. It en- 

 closes itself within its shell by a secretion of calcareous mucus, which, 

 becoming dried, and more or less hardened according to the species, 

 answers the purpose of an operculum. There is, however, no struc- 



