44 



GASTEROPODA. 



Fig. 434. — Pnpa (Dindropiipo) ixtusta 

 (after Dawson), a. Natural size ; b, En- 

 larged ; c, Apex enlarged ; d, Sculpture, 

 maKuifled. Coal-measures. 



to be no reason for framing a new genus (Dendropiqm) for 

 its reception. The Pii.jya Vermilionensis of the Coal-measures 



is a near ally of the preceding. 

 ^ With the exception of these 



little shells, all the fossil species 

 of Pirpa are confined to the Ter- 

 tiary period, commencing in the 

 Eocene. 



In the genus Clansilia the 

 shell is spindle - shaped, coiled 

 into a left-handed spiral (" sinis- 

 tral"), with an elliptical aper- 

 ture, partially contracted by 

 shelly processes. The Clau- 

 silice, 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 Testacella has 

 also been indicated as occurring in the Miocene. 



Fam. 3. Limn^id.e. — Shell well developed, thin, and horn- 

 Aperture simple ; lip sharp. The Zimnceidce are 

 all inhabitants of fresh water, and they are 

 found in flu\datile and lacustrine deposits. 

 They are l^elieved to commence 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 abundance. 



The genus Limnma (fig. 435) includes 

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

 elongated sheUs, with a large body-whorl and an obliquely- 

 twisted columella. The species of this genus commence in 



coloured. 



Fig. 435. — Limnwa 

 pyramidalis. Eocene. 



