126 PNEUMONOBRANCHIATA. 



perforated, from Succinea and Bulimus by the axis 

 being depressed (and not elongate), making the shell 

 subglobose or depressed. 



The animals, at the approach of winter, or in very 

 dry weather in summer, recede into their shell, and 

 secrete a quantity of mucus, which being moulded, as 

 it were, on the retracted part of the mantle which 

 encloses the folded-up foot, forms when it dries by 

 exposure, a cover to the aperture, which is usually 

 membranaceous, with a triangular perforation over the 

 respiratory hole of the mantle. 



In some species, as Helix Pomatia, the membrane 

 becomes strengthened with a quantity of calcareous 

 matter, which is first deposited on the triangular spot 

 before referred to. In this case, the animal forms se- 

 veral membranaceous coverings, a little distance from 

 one another, within the outer, hard, calcareous, 

 one ; similar to the membranaceous covering of other 

 Helices. On the approach of warm or damp weather, 

 the animal softens the adhesion that has taken 

 place between the lid and the edge of the mouth of its 

 shell, by emitting a small quantity of fluid mucus ; 

 and the cover is thus easily thrown off by the pressure 

 of the foot. When another is required by external cir- 

 cumstances, the process is commenced afresh. This lid 

 gives the name Pomatia to our largest snail. Lister 

 called the lid the operculum saliva confectum ; Miiller 

 calls it the operculum liybernum, or winter lid ; and 

 more recently it has been named by Draparnaud the 

 epiphragm : the latter name has been generally adopted. 

 Montagu has been blamed for calling it an liybernacu* 

 lum, but this arises from a mistake. Montagu in- 

 tended bv the latter name the hole in which the 



