GASTEROPODA. 



267 



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. 237), discovered by Dr Dawson 

 in the Coal-Measures of Nova 

 Scotia, in the hollow trunk of an 

 erect Sigillaria. This ancient 

 form is remarkably like some 

 living " Chrysalis-shells," and 

 there appears to be no reason 

 for framing a new genus (Den- 

 dropupa} for its reception. With 

 the exception of this little shell, 

 all the fossil species of Pupa are 

 confined to the Tertiary period, 

 commencing in the Eocene. 



In the genus Clausilia the 

 shell is spindle-shaped, coiled 

 into a left-handed spiral (" sin- 

 istral"), with an elliptical aper- 

 ture, partially contracted by 

 shelly processes. The Clausi- 

 lice, so far as known, date their 

 existence from the Eocene 

 Tertiary. 



FAM. 2. LIMACID.E : Shell rudimentary, usually internal or 

 concealed by the mantle. The " Slugs " are included in this 

 family, and they are only known in the fossil state by doubtful 

 remains in the Miocene and Pliocene Tertiary. A species of 

 Testacdla has also been indicated as occurring in the Miocene. 



FAM. 3. LIMN^EID^E : Shell well developed, thin and horn- 

 coloured. Aperture simple ; lip sharp. The 

 Limnaidcs are all inhabitants of fresh water, 

 and they are found in fluviatile \and lacus- 

 trine deposits. They are believed to com- 

 mence in the Jurassic period, members of 

 this family having been described from the 

 Lias and from the Purbeck beds (Upper 

 Oolites). It is not, however, until we reach 

 the base of the Cretaceous system (Weald 

 Clay) that these forms appear in any abun- 

 dance. 



The genus Limtuea (fig. 238) includes the 

 so-called " Pond-snails," characterised by their thin, spiral, 



Fig. *y].Puj>a (_Dendropupa) vetusta. 

 (After Dawson.) a Natural size; En- 

 larged ; c Apex enlarged ; d Sculpture, 

 magnified. Coal-Measures. 



