CLASSIFICATION. 11 



Continental naturalist), of which family, however, 

 we have but a single genus and species, the well- 

 known "zebra mussel." 



The aquatic univalves are either Pectinibranchiata 

 that is, having comb-like gills; or Pulmonobran- 

 chiata those with lung-like gills. In the former 

 order there are three families, which take their 

 names from some of the genera which they contain 

 i.e., Neritidce, Paludinidce, and Valvatidce; in the 

 latter order, one large family, Limnceida, con- 

 tains the genera Limncea, Physa, Planorbis, and 

 Ancylus. 



The terrestrial univalves comprise three families, 

 Limacidce, Testacellidtf, and Helicidce, which in 

 plain English are known as the slugs, the shell- 

 slugs, and the snails, with two much smaller families, 

 to one of which no English name has been given, 

 and which have been formed each for the reception 

 of a single genus (Carycliium and Cyclostoma) of 

 peculiar character. 



All this may appear very dry to the unscientific 

 reader ; but it is scarcely to be dispensed with, since 

 without a system of some sort it would be not only 

 impossible to arrange and store one's facts, but 



