VERTIGO. 167 



of the substance of the shell, caused by some with- 

 drawing of the mantle of the animal in the part im- 

 mediately in connection with them ; this is also the 

 case with many of the foreign " toothed " Helices 

 (^HelicodontcB). The true teeth must be formed 

 nearly in the same way ; but they are produced by 

 repeated deposits of layers of calcareous matter, one 

 over the other, to fill up the cavity as the mantle is 

 withdrawn, while these plaits are produced by a 

 sudden contraction of the part which forms a mould 

 for the newly deposited portion of the shell. 



11. Vertigo Midler. .(Whorl Shell.) 



The animal, like Bulimus, with only two elongate 

 clavate tentacles, the lower pair being wanting 

 or rudimentary, and with a subcylindrical, 

 abruptly obtuse shell, with close-pressed, gradu- 

 ally enlarging whorls ; the mouth contracted, 

 more or less angular, generally toothed in- 

 ternally, and thickened by an exterior rib; 

 peristome simple. 



This genus has a shell very like the PupcB, but 

 was separated from them by Miiller ; and his division 

 has been adopted by most succeeding zoologists, be- 

 cause the animal has the upper pair only, of tentacles 

 which bear the eyes, developed. 



a. Shell dextral, cylindrical ; mouth margined ex- 

 ternally. (Isthmia Gray. Alasa Jeffreys.^ 



74. 1. Vertigo edentula. Toothless Whorl Shell. 

 — Shell conic-oval, ventricose, brown, with five 



M 4 



