VERTIGO. 
167 
of the substance of the shelly 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withdrawn^ while these plaits are produced by a 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 Muller. (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 Pupce, but 
was separated from them by Muller ; 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. Algea Jeffreys.^ 
74. 1. Vertigo edentula. Toothless Whorl Shell. 
— Shell conic-oval, ventricose, brown, with five 
