HELIC1D.E. 127 



animal buries itself, as is proved by his use of the 

 term, at p. 407. 



The power of forming this kind of epiphragm, 

 and the thickening of the outer lip, has been consi- 

 dered a peculiar character of the land Mollusca, but 

 it is now known that pond snails (Limnceus and Plan- 

 orbis), when left dry by the evaporation of the water 

 in which they have been living, thicken the edge of the 

 lip, and form a distinct epiphragm. 



Though the British species are not very numerous, 

 it has been thought advisable to divide them into se- 

 veral sections, to facilitate their determination, and 

 also to show the natural groups into which the nu- 

 merous exotic species naturally fall. 



a. TAPADA. 



Shell subglobose, thin, covered with a green periostraca; 

 axis solid, tivisted ; mouth large, toothless ; peristome 

 only slightly thickened ; epiphragm calcareous, convex. 



18. 1. HELIX aperta. The Tapada Snail. Shell 

 subglobose, ventricose, very thin, brownish- 

 green, rather wrinkled ; mouth large. 



Helix aperta. Born, Mus. t. 15. f. 19, 20.; Dilhc, 

 R. S. ii. 946. 



Helix neritoides. Chemn. Conch, ix. t. 133. f. 120. 

 4. 120. 5. 



Helix naticoides. Drap. Moll. t. 5. f. 26, 27. ; 

 Ferus. Hist. t. 11. f. 17. to 21. 



Pomatia Dioscorides. 



Inhab. hedges, among nettles, in Guernsey. 

 Mr. Edward Forbes, on whose authority this species 

 has been added to our Fauna, discovered a single 



G4 



