HETEROPODA AND PULMONIFERA. 43 



of rare occurrence as fossils, occurring chiefly in fluviatile 

 and lacustrine deposits. The genera above mentioned have 

 been found in the Coal-measures, and are the oldest forms 

 of the group. The chief fossil genera are Helix, Bidimus, 

 Achatina, Pupa, and Glausilia. 



In the genus Helix are the ordinary Land-snails (fig. 433), 

 in which the shell is conical, sometimes depressed, or some- 

 times discoidal; the aperture transverse, crescentic or rounded, 

 and the columella perforated or imperforate. The Land- 

 snails, with two exceptions, are all confined, so far as known, 

 to the Tertiary and Eecent periods. The exceptions to this 

 statement are the Zonites priscus (fig. 433), discovered by Dr 

 Dawson in the Coal-measures of Nova Scotia, and the Dcau- 



Fi§. 433. — Zonites (Conulus) jrriscus (after Dawson), a, Specimen enlarged twelve diameters 

 b, Sculpture, magnified. Coal-measures, Nova Scotia. 



sonella Meeki of the same formation. The former is a true 

 Land-snail referred to Zonites or Gonulus, a sub-genus of 

 Helix itself. 



In Bidimus the shell is turreted or oblong, the columella 

 generally simple, and the outer lijD usually expanded and 

 thickened. In the nearly allied genus Aehatina the colu- 

 mella is twisted, and the lips of the shell-aperture are thin. 

 Both genera date their existence from the Eocene Tertiary. 



In the genus Pupa the shell is cylindrical or oblong, with 

 a round, often toothed, aperture. The oldest member of 

 this genus is the Pupa vetusta (fig. 434), discovered by Dr 

 Dawson in the Coal-measures of Nova Scotia, in the hollow 

 trunk of an erect Sigillaria. This ancient form is remark- 

 ably like some living " Chrysalis-shells," and there appears 



