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. umbilv- 

 cata be minute, the P. pygmcea is exceedingly 

 minute, being not more than a 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 pygmcea. 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. 



