186 PLANORBINA. 
Subfamily PLANORBINE. 
Shell spiral, discoidal or depressed, many- 
whorled; aperture crescentic. 
For description of the animal see Haldeman, 
pal 
Three genera embrace all the American 
species, which, unlike our Phys@, are readily 
recognised, being more sharply defined in their 
characters. These genera are: 
1. Pianorsis, 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. Secmentina, Fleming. Shell orbicular, 
depressed, furnished internally with testaceous 
partitions or teeth. 
(33) 
