CLASSIFICATION. 



97 



genus Ancylus have not been dctcrmmcd by anatomi- 

 cal investigation. The other genera are well kno"\yn, 

 with the exception of one proposed by the author. 



The characters by which the families are distinguish- 

 ed, differ in importance, but the genera contained in 

 them respectively, are so nearly allied by a common 

 peculiarity as to afford obvious and convenient di- 

 \isions. 



The impossibility of arranging animals in an unin- 

 terrupted Linnean series, descending from the most per- 

 fect to those of the simplest organization, has long smce 

 been acknowledged, and every practical attempt to 



VOL. I. 



12 



