Lamarck's Ge^iei'a of Shells. 243 



longitudinal, contracted above. Columella twisted. Right 

 margin very thin, acute, partly projecting beyond the plane of 

 the aperture. No operculum. 



The physse are fresh water shells, thin, brittle and generally 

 reverse. 'Ihey are distinguished from the bullae by their pro- 

 jecting spire, and from the lymnaea, which they otherwise much 

 resemble, by the aperture not being dilated, the right margin 

 projecting a little above its plane. 



Type. Physafontinalis*. (Bulla fontinalis. Lin?i.) 



Shell reverse, oval, diaphanous, smooth, horn-brown, (lutco- 

 cornea ;) spire very short, sub-acute. PI. VII. Fig. 142. — 

 4 Species. 



3. Lymnaea. 



Shell oblong, sometimes turrited, often rather ventrlcose be- 

 low, generally thin ; spire projecting. Aperture entire longi- 

 tudinal. Right margin acute, its lower part, turning to the 

 left and ascending, passes over the columella towards the aper- 

 ture, forming a very oblique fold. No operculum. 



The very oblique fold on the columella, distinguishes the 

 lymnaea from the bulimus, and the regular uninterrupted plane 

 of the aperture, from the physa. 



Type. Lymncea stagnalis \. (Helix stagnalis. Linn.) 



Shell acute-ovate, ventrlcose, thin, transparent, longitudinally 

 substriated ; reddish grey ; last whorl subangular above ; spire 

 conico-subulate ; aperture large ; lip broad. 



France. PI. VII. Fig. 143. 12 Species. 

 3rd Family. 

 Melaniana. (3 Genera.) 



Operculated, fluviatile trachelipoda ; breathe only water. Two 

 tentacula. Operculum horny. 



Borders of the aperture of the shell disunited ; right margin 

 always acute. 



Most of the shells of this family are exotic, and are covered 

 with a brownish green or blackish epidermis. 



• Of the springs. Lamarck's second species ; his type is P. castatiea. 

 ( Of slngitnnt waters. The first species, L. columnnr'is, Laroarck has re- 

 moved to the genus Achatina — See erratum. Vol. vii. p. G78. 



