CLASSIFICATION. 
97 
genus Ancylus have not been determined by anatomi- 
cal investigation. The other genera are well known, 
with the exception of one proposed by the author. 
Pueumobranchiate 
Mollusca. 
SECT. I. 
Terrestrial. 
Living upon 
the land. 
sect. n. 
Aquatic. 
Inhabiting 
the waters. 
FAMILIES. 
GENERA. 
Lirnacidre . > 
Vaginulus. 
Tebennopliorus 
Arion. 
Limax. 
Helicidse . • 
' Vitrina. 
Succinea. 
Helix. 
Bulimus. 
Glandina. 
Pupadoe . . . 
' Cylindrella. 
Pupa. 
Vertigo. 
Helicinadse . 
Helicina. 
Cyclostoma. 
Limneadae . « 
• 
Limnea. 
Planorbis. 
Physa. 
Ancylus. 
1 Auriculadfe . 
Auricula. 
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- 
visions. 
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 since 
been acknowledged, and every practical attempt to 
vol. i. 26 
