242 Lamarck's Genera of Shells. 



by the outward reflection of the margin of the adult shell, whilst 



in that genus it is always sharp, and not reflected; from the 



pupa, by the regularity of the aperture, which is never angular. 



Type. Cyclostoma volvulus *. 



Shell trochiform, deeply umbilicate ; transversely striated ; 



variegated with brownish white and red ; spire acuminated ; 



aperture white, or brownish ; margin reflected. PI. VII. Fig. 



140. 28 Species, the two last doubtful. 



2d Family. 



Lymnjeana. (3 Genera.) 



Amphibious trachelipoda, generally provided with an oper- 

 culum, and two flattened tentacula without eyes at their sum- 

 mit. They live in fresh water, and come to the surface to 

 breathe air. 



Shell spirivalve, generally smooth externally ; right margin 

 of the aperture always acute and not reflected. 

 1 . Planorbis t. 



Shell discoidal, spire flattened, scarcely projecting; all the 

 whorls visible on both sides. Aperture oblong, lunate, very 

 distant from the axis of the shell ; margin not reflected. No 

 operculum. 



Fresh water shells, generally thin, brittle, diaphanous ; the 

 whorls of some are subcylindrical, of others carinate or angular. 

 Aperture sublongitudinal, with an internal projection formed 

 by the penultimate whorl. 



Type. Planorbis corneus J. (Helix cornea. Linn.) 



Shell opaque, flat depressed above, broadly umblicate below ; 

 horn or chestnut brown ; whorls transversely striated. 



France. PI. VII. Fig. 141. 12 Species. 

 2. Physa§. 



Shell convolute, oval or oblong, spire projecting. Aperture 



• A twistiiig. t From planus, flat, and orbis, an orb. 



t Homy. Laraarck'.s second species. His type, P. comu arietis, Mr. 

 Sowerby considers as an Ampullaria. 



4 Mr Sowerby is of opinion, tliat there is not sufficient ground for forming 

 distinct crenera, of this and the following shell. He has therefore struck out 

 physa, and placed it with the genus lymna^a. (Genera of recent and fossil 

 shells. No. 8.) 



