52 INTRODUCTION. 



cations ; the genera are Helix, Carocolla, Anostoma, 

 Helicina, Pupa, Clausilia, Bulinus, Achatina, Succinea, 

 Auricula, Cyclostoma. 



2. Lymneana (plates, fig. 308 to 312). The shells of this 



family are found in fresh water, wells, ditches, and 

 ponds. They are of a light horny structure, and simple 

 form. The genera Planorbis, Physa, and Lymnea are 

 placed in this family by Lamarck. 



3. Melaniana (plates, fig. 313 to 317). These are also 



found in fresh water, principally in rivers; they are 

 thicker than those of the last family ; and the greater 

 part of them have elevated spires composed of numerous 

 whorls. This family contains the genera Melania, 

 Melanopsis, Pirena. 



4. Peristomata (plates, fig 318 to 322). These are also 



fresh-water shells, having opercula, and covered by a 

 smooth green, or greenish-brown epidermis. They differ 

 from the last family in having the peritreme entire. 

 The genera are Valvata, Paludina, and Ampullaria. 



5. Neritacea (plates, fig. 323 to 333). The peculiarity of 



the shells of this family consists in the inner lip being 

 flattened and rather straight at the inner edge. The 

 genera are Navicella, Neritina, Nerita, Natica, and Jan- 

 thina, the last of which forms an exception to the general 

 character, and is placed by De Blainville in a family by 

 itself. 



6. Macrostomata (plates, fig. 334 to 341), so named, on 



account of the large open aperture which they present 

 in comparison to the spire. The shells of this family, 

 which contains the genera Stomatia, Stomatella, and 

 Haliotis, are pearly within. 



7. Plicacea (plates, fig. 342 to 344), contains the genera 



Tornatella and Pyramidella. 



8. Scalariana (plates, fig. 345 to 352). The genera Ver- 



