Shells of Common Occurrence. 149 
Wherever limestone or chalk abound, there it is 
found, whether in the cracks of old walls, amidst 
ivy, under stones and the bark of trees, or on the 
sea-cliffs and valleys. If, however, the P. wmbil- 
cata be minute, the P. pygmea is exceedingly 
minute, being not more thana line in length. By 
most authors this tiny shell, found, though not 
abundantly, in all parts of Great Britain and Ire- 
land, both in wet and dry situations, but principally 
in dry, is classed as Vertigo pygmea. The P. 
substriata, so called from its cylindrical, shining, 
polished little surface being streaked longitudinally, 
is much the same size, a line long, half a line broad, 
and though rare where it is found, is at the same 
time widely diffused from Cornwall, Devonshire, 
and Suffolk, to Preston, Lancaster, and Newcastle- 
upon-Tyne. And speaking of Newcastle-upon- 
Tyne, it ought to be mentioned that Mr. Joshua 
Alder, the great molluscous authority and genius 
loci, has pointed out a remarkable structure in the 
interior of the pupa, the use of which has not yet 
been ascertained. It consists of a raised thread- 
like laminar process, winding spirally round the 
columella, and similar lamina running spirally on 
the upper side of the volutes, with small flat 
transverse plaits at intervals in the interior. 
