186 PLANORBIN.E. 



Subfamily PLANORBIJS^. 



Shell spiral, discoidal or depressed, many- 

 whorled; aperture crescentic. 



For description of the animal see Haldeman, 



p.i. 



Three genera embrace all the American 

 species, which, unlike our Physce, are readily 

 recognised, being more sharply defined in their 

 characters. These genera are : 



1, Planorbis, Guettard. Shell dextral, 

 discoidal; spire depressed, whorls numerous, 

 visible on both sides; aperture crescentic or 

 transversely oval, unarmed; peristome thin, 

 incomplete, the upper margin produced. 



2. Segmentina, Fleming. Shell orbicular, 

 depressed, furnished internally with testaceous 



partitions or teeth. 



(33) 



