• COMMONER FOEMS OF SHELLS. 15 



ata — are all of small size and gregarious habits, living 

 in moss or amongst stones and roots. Besides their 

 variation in size and colour, they are to be distin- 

 guished by the curious processes called " teeth," 

 which fence in and contract the mouth of the shell. 



The eight or nine species which compose the 

 closely allied genus Vertigo are but miniature forms 

 of Pwpa, and their habits are similar. The chief 

 difference is to be found rather in the animal than 

 in the shell, the inhabitant of which has two tentacles 

 instead of four, as in Pupa. 



If we now restore the pointed apex which we 

 cut off from Bidimus to model Pupa, and, compress- 

 ing the shell laterally, elongate it until the length is 

 five or six times greater than the width, we have 

 something like ClausiUa before us. It is a spindle- 

 shaped shell, with a longer spire than BuUmus, and 

 is especially characterized by having a curious inter- 

 nal process called the "clausilium" (hence the name), 

 which acts as a valve or lid in closing the shell 

 against all intruders. There are four species to be 

 met with in the British Islands — C. hiplkata, lam- 

 inata, rugosa, and Fiolphii — all of which frequent 

 the trunks and roots of trees, and may be found also 



