Lamarck's Genera of Shells. 245 



4th Family. 

 Peristomiana. (3 genera.) 



Operculated, fluviatile trachelipoda ; breathe only water. 

 Shell operculated, conoidal, or subdiscoidal ; borders of the 

 aperture united. 



The shells of this family all belong to fresh water, and have 

 a thin greenish or brown epidermis. They are distinguished 

 from the three preceding genera by the margin of the aperture 

 being united. 



1. Valvata*. 



Shell discoidal or conoidal ; whorls cylindrical ; spiral 

 cavity complete, or not deranged by the penultimate whorl ; 

 aperture rotundate ; margins united, acute ; operculum orbi- 

 cular. 



Type. Valvata pisciyialisf. 



Shell globose-conoidal, subtrochiform, perforate, whitish ; 

 about five whorls ; apex of the spire obtuse. 



France. PI. VII. Fig. 147. 4 species are known, but La- 

 marck only describes the above. 



2. PaludinaJ. 



Shell conoidal, whorls rounded or convex, modifying the 

 spiral cavity ; aperture rounded oval, longitudinal, angular at 

 the summit ; the two margins united, acute, never curved out- 

 wards ; operculum orbicular, horny. 



The paludinse generally live in fresh water, though some 

 inhabit brackish, and even salt, water. They are distinguished 

 from the valvatae by the somewhat elongated and angular form 

 of the aperture. 



Type. Paludina vivipara§. (Helix vivipara, Zmw.) 



Shell ventricose-conoidal, thin, diaphanous, very delicately 

 striated longitudinally, brownish green ; obsolete, brown-red, 

 transverse bands ; five whorls, rotundate, turgid ; sutures very 

 marked. 



France. PI. VII. Fig. 148. 7 species. 



j_« Vahn,i\ftililingdom\ortakc. t OJ Ike pooln. 



t From [lalus, a marsh. § Viciporoxis 



