8l2 DIVISIONS OF THE GASTROPODA. 



occurring also in deposits of Secondary age, and even in rocks as 

 old as the Carboniferous formation. If JDawsonella be regarded as 

 a member of the Helicinidce, (see p. 778), the oldest representatives 

 of the Helicidce are the Zonites (Conu/us) priscus and the Pupa {Den- 

 dropupd) vetusta of the Coal-measures of North America, both of 

 which were first discovered and described by Sir William Dawson. 



In the genus Helix are the ordinary Land-snails, of which about 

 two thousand living species are known, belonging to a large number 

 of subgeneric groups. The shell in Helix is very variable in shape, 

 sometimes conical, sometimes depressed, and sometimes discoidal ; 

 the aperture being transverse, crescentic or rounded in shape, and 

 the columella being perforated or imperforate. The most ancient 

 of the typical Helices appear in the Eocene rocks, and a large num- 

 ber of Tertiary species have been recognised. 



In Zonites the shell is thin, usually hyaline, in the form of a 



Fig. 726. — Zonites (Conulus) firiscus (after Dawson), <z, Specimen enlarged twelve diameters; 

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



depressed spiral, with a simple and sharp-edged peristome, and 

 almost always umbilicated. The oldest example of this genus is 

 the Z. priscus (fig. 726) of the Coal-measures of Nova Scotia. Other 

 species occur in the later Tertiaries ; and the Archmozonites of the 

 Oligocene and Miocene deposits is nearly related to Zonites. 



In Bulimus the shell is oval or turreted, the columella is straight, 

 the aperture is oblong, and the outer lip is expanded and thickened. 

 Bulimulus includes forms with a shell very similar to that of Buli- 

 mus, but with a generally thin lip and an elongated aperture. The 

 earliest forms of Bulimus appear in the Upper Cretaceous rocks. 

 In the genus Achatina the shell is like that of Bulimus, but the 

 columella is twisted. No undoubted fossil representatives of this 

 genus have been recognised. 



In the genus Clausilia the shell is spindle-shaped, and is. coiled 

 into a left-handed spiral, the aperture being elliptical, and partially 

 contracted by two folds of the inner lip. The mouth of the shell 



